The ceremony drew older residents as well as children wearing commemorative shirts and stickers marking the terror attack of 2001, long before they were born.

Two former New Yorkers in the audience who have relocated to Florida, Loretta Grande, 60, and Kristen DeRose, 48, remarked on how differently the anniversary is remembered in the two states.

“Here in Florida, it’s like it was never real,” Ms. Grande said. “That’s heartbreaking for us, because we know it was real. We’ve seen the devastation of the aftermath of it.”

Reporters on the street near Saturday’s march were kept away from the women by Taliban fighters armed with automatic rifles and were not allowed to speak with any of them. Later attempts to reach the participants through social media or the university went unanswered.

Held on the 20th anniversary of the Sept. 11 attacks, the demonstration served as a stark reminder of how the women of Afghanistan could be thrown back to an earlier era following last month’s Taliban takeover and departure of American troops from the county.

Since the United States and its allies departed Kabul on Aug. 30, leaving Afghanistan under the control of the Taliban, the country’s women have been at the forefront of protests demanding that their rights continue to be respected.

Taliban leaders have responded to those protests with violence, beating participants, including women, and insisting that anyone taking to the streets for a public demonstration must first be granted approval from their caretaker government.

But over the years, another wave of victims has emerged: emergency workers who were stricken with 9/11-related illnesses from being exposed to toxins near ground zero in the aftermath of the attacks.

“Tragically, the number of children who lost parents because of 9/11 continues to grow as more and more first-responders lose their lives due to illnesses caused by breathing toxic air during the rescue efforts,” Mr. Clinton said in recorded remarks.

Now the fund is being expanded to assist their children, with the launch last week of a campaign to raise an extra $25 million in academic assistance, largely for students whose parents have died because of exposures during recovery efforts.

Large donors include the Ralph Lauren Corporate Foundation and Medal Dash, which raised $115,000 from a run with more than 8,000 participants, fund officials said.

The 9/11 attacks had changed many young people’s lives, she said, and her son wanted to protect his country. So in 2003, he joined the Marines, who sent him to Iraq, then Afghanistan.

Ms. Malachowski, 65, said she was scared at first, but also proud of her son.

She showed a photo of her son in Afghanistan, standing with children. Everyone was smiling. It was taken not long before her son’s death, from an explosive device. “Everybody came back from the explosion, but why didn’t my Jimmy?” she asked.

She said that many parents had experienced terrible pain, and questioned the need for so much loss.

Then Ms. Malachowski pulled out small American flags and placed them near other graves — the graves of the children of other families she has met over the years.

The ceremony honored and remembered the 2,983 people killed in the attacks at the World Trade Center site, the Pentagon, aboard Flight 93 and in the Feb. 26, 1993 World Trade Center bombing. Their names are etched in bronze above pools at the 9/11 Memorial.

The ceremony is an annual tradition that was altered because of the coronavirus pandemic last year, when prerecorded audio of the names was played. The event held on Saturday more closely resembled those of years past.

After a 21-gun salute and speeches from Gov. Pete Ricketts and Lincoln’s fire chief, David Engler, the audience applauded as the 61 members of Nebraska Task Force One, a search and rescue team that was deployed to New York in 2001, were honored. Some task force members rose as their names were called.

As families filed out after the ceremony, a young boy reached out to a police officer, who gave him a a high-five.

Two miles away, at the Veteran’s Memorial Garden, Battalion Chief Mark Majors of Lincoln’s Fire and Rescue Department, who was at the World Trade Center after 9/11, said the day served as “an opportunity to educate and explain to an entire generation the promise that so many made 20 years ago.”

“This is the only place I come,” he said. “There’s no speeches. There’s no politics involved. We read the names, we honor the fallen. It’s a simple tribute, which I like.”

He was one of the hundreds of firefighters, from New York and beyond, who crowded along Riverside Drive on Saturday to pay tribute to the colleagues and friends lost on 9/11.

“We lost a couple of guys there, good guys. You think about them often,” said Derek Harkin, 55, a battalion chief in Brooklyn and 27-year veteran of the Fire Department. “It seems like yesterday, but it feels like a long time ago.”

Not everyone in the crowd was a firefighter. Candy Dato, 73, wiped tears from her eyes as she remembered her experiences on 9/11, down to the shade of the sky.

“I, for whatever reason, have a vivid memory: it was such a bright, beautiful blue,” said the retired nurse. “No one’s forgotten.”

Hundreds of firefighters from across the country mingled in front of the memorial, greeting each other with laughs and hugs that punctuated the solemnity of the occasion. They came from Miami, Oakland, Chicago, and as far away as France. But as soon as the ceremony started, they all lined up in perfect unison, shoulder to shoulder, united by their uniforms.

The Biden administration has used the solemnity of the day to plead for Americans to view what happened on Sept. 11 as a useful lesson for the current political and cultural divisions wrought, in part, by the coronavirus pandemic. Ms. Harris, memorializing the 40 passengers and crew members aboard Flight 93 who fought back against hijackers, encouraged Americans to remember their sacrifice.

“On this 20th anniversary, on this solemn day of remembrance, we must challenge ourselves to, yes, look back. For the sake of our children. For the sake of their children,” Ms. Harris said. “And for that reason, we must also look forward. We must also look toward the future. Because in the end, that is what the 40 were fighting for: Their future. And ours.”

“On the days that followed Sept. 11, 2001, we were all reminded that unity is possible in America. We were reminded, too, that unity is imperative in America. It is essential to our shared prosperity, our national security, and to our standing in the world.”

The vice president took the stage after former President George W. Bush; Deb Haaland, the secretary of the interior; and Gov. Tom Wolf of Pennsylvania delivered their own remarks. Mr. Bush, too, stressed the importance of Americans coming together.

“On America’s day of trial and grief, I saw millions of people instinctively grab for a neighbor’s hand and rally to the cause of one another,” Mr. Bush said. “That is the America I know.”

The event in Chicago was one of 40 stair climbs around the country. Participants climbed or walked the equivalent of the 110 stories of the World Trade Center, each wearing a badge with the name and photo of one of the emergency workers who died in the attacks.

Many firefighters from the Chicago area took part.

Omar Juarez, 23 and a firefighter from Westmont, Ill., was too young to remember 9/11, but he said the events resonated with him nonetheless.

“It could happen to any one of us. It makes you realize how fragile life is,” Mr. Juarez said.

Marisa Price, a 25-year-old firefighter from Westmont, said she found herself thinking about what must have been going through the minds of New York firefighters as they worked to rescue people that day.

“I’ll be thinking of them waking up and going to work like a normal day and then having to face the horrors of what happened, having to walk up those towers,” she said. “A lot of them were my age at the time and made the ultimate sacrifice.”

A dozen 911 dispatchers from Chicago’s Office of Emergency Management also took part in the memorial climb on Saturday.

Brenda Holifield, a dispatcher, said she was on the job 20 years ago, and often thinks of dispatchers who took calls that day.

“I can feel how they felt,” said Ms. Holifield, 47. “Your heart goes out because they must have felt helpless but stayed calm and did their job.”

Before the climb began, members of the Chicago Fire Department presented the colors, bagpipers played “Amazing Grace,” and Talia Martino, a 17-year-old senior from James B. Conant High School in suburban Hoffman Estates, sang the national anthem.

Once an active runner, she developed breathing problems after she was exposed to toxic burn pits in Kandahar, Afghanistan. Watching Kandahar, where she had tried to make a difference, and then the entire country quickly fall to the Taliban exacerbated her pain.

“It all feels like a complete failure,” she said from her home in Washington, D.C. “I have my own demons from my time there, and I worry about other veterans and the defeat they must be feeling.”

Some veterans are wondering if the wars were worth it, said Bonnie Carroll, the founder of the Tragedy Assistance Program for Survivors, a support organization for those grieving the death of a service member.

“In World War I and World War II, if you died, you most likely died on the battlefield,” she said. “But many of our loved ones are now bringing the war home with them and dying from suicide as a result of post-traumatic stress or illness as a result of exposures.”

U.S. officials estimate that more than 3.5 million service members who deployed were exposed to toxic smoke from the roughly 250 pits used in Iraq and Afghanistan. President Biden has said that he believes toxic substances from burn pits contributed to the brain cancer of his son Beau, who served with the Delaware Army National Guard at Balad Air Base in Iraq and died of the illness in 2015.

“For those too young to recall that clear September day, it is hard to describe the mix of feelings we experienced,” Mr. Bush said at a ceremony held at the memorial. “There was horror at the scale of destruction, and awe at the bravery and kindness that rose to meet it. There was shock at the audacity of evil, and gratitude for the heroism and decency that opposed it.”

He added that “the actions of an enemy revealed the spirit of a people, and we were proud of our wounded nation.”

Recalling how the American people responded to the Sept. 11 attacks, Mr. Bush painted a starkly different picture compared with the embittered politics of the present day.

“In the weeks and months following the 9/11 attacks, I was proud to lead an amazing, resilient, united people,” he said, though many Muslim Americans reported facing heightened discrimination in the wake of the attacks. “When it comes to the unity of America, those days seem distant from our own.”

He continued: “Malign force seems at work in our common life that turns every disagreement into an argument, and every argument into a clash of cultures. So much of our politics has become a naked appeal to anger, fear and resentment. That leaves us worried about our nation and our future together.”

“I come without explanations or solutions,” Mr. Bush added. “I can only tell you what I’ve seen. On America’s day of trial and grief, I saw millions of people instinctively grab for a neighbor’s hand and rally to the cause of one another. That is the America I know.”

Eight months after the Capitol was stormed by supporters of President Donald J. Trump, Mr. Bush warned against domestic extremism, saying that “the dangers to our country can come not only across borders but from violence that gathers within.”

“There is little cultural overlap between violent extremists abroad and violent extremists at home,” he continued. “But in their disdain for pluralism, in their disregard for human life, in their determination to defile national symbols, they are children of the same foul spirit, and it is our continuing duty to confront them.”

In his remarks, Mr. Bush paid tribute to the passengers and crew members of Flight 93, which crashed in a field after those on board fought back against the hijackers and diverted them from their intended target. “Many who are now alive owe a vast, unconscious debt to the defiance displayed in the skies above this field,” he said.

In the wake of the Sept. 11 attacks, Mr. Bush led the nation into the war in Afghanistan and, later, the war in Iraq. Speaking less than two weeks after the last American troops left Afghanistan, he acknowledged in his remarks that military actions over the past two decades “have led to debate.”

He offered a message for veterans, saying they had been “a force for good in the world” and adding, “Nothing that has followed, nothing, can tarnish your honor or diminish your accomplishments.”

In Britain, Prime Minister Boris Johnson paid tribute to the victims of the attacks in a video address played at a memorial event at the Olympic Park in East London.

Mr. Johnson said that while the threat from terrorists remained, the last 20 years had shown that “they failed to shake our belief in freedom and democracy; they failed to drive our nations apart, or cause us to abandon our values, or to live in permanent fear.”

In a message to President Biden, Queen Elizabeth II said, “my thoughts and prayers — and those of my family and the entire nation — remain with the victims, survivors and families affected, as well as the first responders and rescue workers called to duty.”

Jacinda Ardern, New Zealand’s prime minister, who was in the United States on Sept. 11, 2001, described it as a “day I’ll never forget.”

In a statement, Ms. Ardern said: “I saw first hand the shock and fear that goes hand in hand with terrorism.”

Many world leaders took to Twitter to commemorate the attacks, including the prime ministers of Ireland, Greece and Canada — Micheál Martin, Kyriakos Mitsotakis and Justin Trudeau — and the presidents of France and South Korea, Emmanuel Macron and Moon Jae-in.

“20 years have passed, but the shock of that day still remains as deep wounds in the hearts of so many,” Mr. Moon said.

In Madrid, staff from the U.S. embassy placed a floral wreath at a memorial in Juan Carlos I Park. From the compound of the U.S. embassy, two giant light beams pierced the night on Friday, shining up into the sky to represent the twin towers.

Flags flew at half-staff outside NATO headquarters in Brussels, where Jens Stoltenberg, NATO’s secretary-general and Douglas D. Jones, the U.S. chargé d’affaires, held a moment of silence at the Sept. 11 memorial — a piece of twisted metal from the World Trade Center — at the precise minute that the first plane struck the North Tower of the complex.

In a tweet, Prime Minister Narendra Modi of India described the 9/11 attack as an “attack on humanity.”

Raphael Minder contributed reporting from Madrid.

United Airlines Flight 175 departed from Logan for Los Angeles at 7:58 a.m. on the morning of Sept. 11 with 65 people aboard. Five of them were hijackers.

American Airlines Flight 11, with 92 people aboard, including five hijackers, took off a minute later.

Within 65 minutes, both planes had struck their targets.

The reading of the names was followed by remarks from Pentagon leadership, who spoke at the base of the building before a flag draped over its walls.

“They were irreplaceable to their families, instrumental in their jobs, woven into the fabric of their community,” General Milley said, speaking before dozens in attendance.

Some people who survived the attack on Sept. 11 still work in the building 20 years later.

“The hallways that we tread were the ones where so many of them walked. It will always be our duty to fulfill their missions and live up to their goodness and to stand guard over this democracy,” said Mr. Austin, his voice wavering with emotion. “We still work here. We still remember.”

In an interview on CNN shortly after the ceremony, Mr. Austin said he mourned the loss of those who died on Sept. 11, 2001 and recalled being stationed at Fort Stewart, an Army base in Georgia on that day.

“I was watching this airplane as it crashed into one of the towers there,” Mr. Austin said. “We suddenly realized that we were under attack.”

Mr. Austin also reflected on the war in Afghanistan, which President Biden ended last month by withdrawing American forces who had been on the ground for 20 years to prevent the country from becoming a haven for terrorist groups. The United States ultimately failed to defeat the Taliban, which allowed Al Qaeda, the group responsible for the Sept. 11 attacks, to operate in the country.

“We significantly degraded the Al Qaeda network. I would point to you the fact that no one has attacked the United States, and especially from that region, in 20 years,” Mr. Austin said, adding that the United States still retains the ability to launch counterterrorism strikes in Afghanistan.

Asked about the veterans who feel anguish at what became a haphazard withdrawal from the country where more than 2,000 U.S. service members died, Mr. Austin said “it will take time to work through this.”

“We’ve told our allies and what we tell our own people is that we’ll continue to work together,” Mr. Austin said. “That’s who we are. That’s America.”

Like relatives of most of the other victims, Ms. Morgan had submitted a reference sample nearly two decades ago of her mother’s DNA — so long ago, she does not recall what it was. But through new technology, the medical examiner’s office matched her sample to a tiny bone fragment found amid the thousands of remains.

“I didn’t know they were still attempting that after all these years, that it was something that was ongoing,” Ms. Morgan said. “At this point, what is it that you’re sifting through?”

Her mother, Dorothy Morgan, became the 1,646th World Trade Center victim to be identified through DNA testing. She was working as an insurance broker in the North Tower of the World Trade Center when she disappeared in the rubble. Remarkably, the 1,647th match came days later: a man whose name the agency did not release in accordance with his family’s wishes.

They were the first positive identifications since 2019. Victim identifications come less than once a year today, a far cry from the years immediately following 2001, when there were hundreds of identifications each year.