Three people have died and three more have been seriously injured after stabbings in a park in Reading, police confirmed.
Police have launched a murder investigation and a 25-year-old man from Reading was detained at the scene.
The attack happened at Forbury Gardens at about 19:00 BST and several people were stabbed.
Police are not currently treating the incident as terror-related, but counter terrorism officers were called.
Security sources have told the BBC a man arrested at the scene is thought to be Libyan.
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” the suspect to the ground, according to the Sunday Mirror.
Detective Chief Superintendent Ian Hunter, from Thames Valley Police, said they are not looking for any other people in connection with the attack.
He added: “This is not currently being treated as a terrorism incident, however officers are keeping an open mind as to the motivation for the incident and are being supported by colleagues from Counter Terrorism Policing South East.”
Sources earlier told the BBC that police were looking at terror as a possible motivation for the attack.
About a dozen armed police officers carrying shields were later seen entering a block of flats in Basingstoke Road in Reading at around 23:00 BST.
BBC News Home Affairs correspondent Dominic Casciani, who was outside the block of flats, said he heard a loud bang at the scene.
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.
South Central Ambulance Service said it was “assessing and treating a number of casualties who had sustained injuries as a result of the incident”.
‘He started coming towards us’
Laurence Wort, 20, who was visiting Reading for the day, told the BBC he was about 10m away when the attack started.
“The park was pretty full, a lot of people sat around drinking with friends when one lone person walked through, suddenly shouted some unintelligible words and went round a large group trying to stab them.
“He stabbed three of them and then turned and started running towards me, when we turned and started running.
“When he realised that he couldn’t catch us he tried to stab another group sat down.
“He got one in the back of the neck and then when he realised everyone was starting to run, he ran out the park.”
Claire Gould, a freelance journalist who lives in Reading, said she walked past Forbury Gardens at around 18:40 BST “and everything seemed calm”.
She then saw an air ambulance land in King’s Meadow – another park close to the scene – at around 19:00 BST, followed by a second around 10 minutes later.
“There were multiple sirens from 19:00 going on for the next couple of hours and police helicopters [were] circling,” she added.
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 the incident was not 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 organiser Nieema Hassan said in a video message posted to Facebook: “They [BLM protesters] 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.”