At least three people have died and others injured after a serious stabbing in a park in Reading, sources say.
Security sources have told the BBC a man arrested at the scene is thought to be Libyan.
Counter terrorism officers are at the scene, but the motive for the attack has not yet been confirmed.
The attack happened at Forbury Gardens at about 19:00 BST but neither the number of injured or fatalities have been officially confirmed by police.
One eyewitness told the BBC he saw a man move from group to group stabbing people in the park.
There were reports that a police officer “rugby tackled” a suspect to the ground, according to the Sunday Mirror.
Thames Valley Police said a man was arrested at the scene and was now in custody.
Armed police carrying shields were seen entering a block of flats in Basingstoke Road in Reading at around 20:00 BST.
Security sources told the BBC that police were looking at terror as a possible motivation for the attack.
Prime Minister Boris Johnson said his “thoughts are with all of those affected by the appalling incident in Reading”.
“My thanks to the emergency services on the scene,” he added.
Laurence Wort, 20, who was visiting Reading for the day on Friday, told the BBC he was about 10m away from the attack.
“A man walked along the path opposite where we were walking past a group of eight to 10 men, before darting to the left and going to stab three people in the first group.
“Changing direction, he runs towards us and then when we realises he’s not going to catch us, he turned back and went for another group that hadn’t realised what was happening yet, and he managed to stab someone in the second group,” Mr Wort said.
Home Secretary Priti Patel tweeted to say she was “deeply concerned” at the incident.
She said: “My thoughts are with everyone involved, including police and emergency responders at the scene.”
Policing minister Kit Malthouse tweeted his “deepest sympathies to all affected” and described the incident as “horrific” and “dreadful”.
Labour leader Sir Keir Starmer said the stabbing was “very concerning”.
Thames Valley Police said there was no indication the incident was connected to an earlier Black Lives Matter (BLM) protest in the park.
Reading council’s leader urged people to stay away from the area.
A spokesman for Thames Valley Police said: “A number of people sustained injuries and were taken to hospital. A police cordon is in place in Forbury Gardens as officers investigate.
“We are asking the public to avoid the area at this time.”
BLM organisers said the incident was unrelated to a protest march in the area earlier.
Nieema Hassan said in a video message posted to Facebook: “They were very peaceful and we worked with the police liaison.
“In terms of the protest and the people who attended from Black Lives Matter, we’re all safe.
“None of us are affected. We had all left by the time this happened.”