But the electoral alliance of his top rival, former president Robert Kocharyan, swiftly contested the vote results and alleged election fraud.
The vote has been seen as a two-horse race, with both Pashinyan, 46, and Kocharyan, 66, drawing massive crowds in the run-up to the polls.
Preliminary results from Sunday’s polls showed Pashinyan’s party leading with 57% of the vote, far ahead of Kocharyan’s alliance with 19%.
Official results based on ballots from more than 60% of precincts counted showed one other party garnering more than 5% of the vote needed to get seats in parliament.
“The people of Armenia gave our Civil Contract party a mandate to lead the country and personally me to lead the country as prime minister,” Pashinyan announced early on Monday.
“We already know that we won a convincing victory in the elections and we will have a convincing majority in parliament,” he added, urging supporters to turn up at Yerevan’s main square Monday evening.
Kocharyan’s electoral bloc said it would not recognise Pashinyan’s quick claim to victory, which came when just 30% of precincts had been counted.
“Hundreds of signals from polling stations testifying to organised and planned falsifications serve as a serious reason for lack of trust,” the bloc said in a statement, adding it would not “recognise” the results until the “violations” were studied.
Earlier on Sunday evening, the general prosecutor’s office said it had received 319 reports of violations. It said it had opened six criminal probes, all of which concerned bribes during campaigning.
The vote was being followed by Armenia’s Soviet-era master Russia, arch-foe Azerbaijan and Turkey, which backed Azerbaijan in the six-week war over the breakaway region of Nagorno-Karabakh last year.
Despite stifling heat, nearly 50% of around 2.6 million eligible voters cast their ballots, election officials said. Some observers said turnout in the South Caucasus country of three million people was higher than expected.
During a campaign marred by polarising rhetoric, Pashinyan had said he expected his party to secure 60% of the vote. Election officials said the vote was conducted in accordance with Armenia’s legislation.
Kocharyan was himself accused of rigging a presidential election in favour of his hand-picked ally and presiding over a deadly crackdown on protesters in 2008.
Armenia won international praise for holding its first free and fair vote under Pashinyan in 2018.
On the streets of Yerevan on Sunday, Armenians voiced conflicting opinions about Pashinyan. Voter Anahit Sargsyan said the prime minister, who spearheaded peaceful protests against corrupt elites in 2018, deserved another chance. She said she feared the return of the old guard whom she accused of plundering the country.
“I voted against a return to the old ways,” said the 63-year-old former teacher.
Another voter, Vardan Hovhannisyan, said he had cast his ballot for Kocharyan, who calls Russian leader Vladimir Putin his friend.
“I voted for secure borders, solidarity in society, the return of our war prisoners, the well-being of the wounded and a strong army,” said the 41-year-old musician.
Critics blame Pashinyan for having ceded territory in and around Karabakh to Azerbaijan in a humiliating truce agreement, and accuse him of having failed to deliver reforms.
Pashinyan has said he had to agree to the Moscow-brokered peace deal with Azerbaijan in order to prevent further human and territorial losses.
More than 6,500 people were killed in the war, according to the latest official figures from Armenia and Azerbaijan.
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