Three people have died and three more have been seriously injured after stabbings in a park in Reading, police confirmed.
A 25-year-old man from Reading has been arrested on suspicion of murder after being detained at the scene.
The attack happened at Forbury Gardens at about 19:00 BST where 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.
Home affairs correspondent Daniel Sandford said he had been told the man had previously been in prison in the UK for a relatively minor conviction, not a terrorism offence.
One eyewitness told the BBC how he saw a man moving between groups of people in the park, trying to stab them.
There were reports a police officer had "rugby tackled" the suspect to the ground, according to the Sunday Mirror.
Det Chf Supt Ian Hunter, from Thames Valley Police, said police were not looking for any other people in connection with the attack, adding that officers were "keeping an open mind as to the motivation for the incident".
Police have asked anyone with mobile phone footage of the attack to come forward and people have been urged not to share videos and photos online "out of respect for the victims' families".
Thames Valley Police said the incident was not connected to an earlier Black Lives Matter (BLM) protest in the park.
At 20:00 BST, officers arrived at a block of flats in Basingstoke Road in Reading and secured the scene, leaving residents waiting outside.
More than a dozen armed police officers carrying shields were seen entering the block at about 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 about an hour and a half later, after which several armed officers left.
At the scene
By Linda Serck, BBC News
Reading appears desolate and in mourning this morning. Large areas outside the gardens are taped up and there are not many people here, mainly journalists and their TV cameras.
A strong wind is rattling litter in grey, deserted streets, most of which are being patrolled by armed police. Commuters are struggling to get to work as so many roads have been cordoned off with police tape.
The few people arriving from Reading station expressed "shock". Marie Castro from Slough works at a Costa coffee shop in Reading and said: "I was scared to be here but I have to be here for work."
The attack "doesn't seem right for Reading", she added.
"It's multicultural and really friendly. I was really shocked when I heard the news".
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.
Emergency air ambulances were called to the scene and five ambulance crews from South Central Ambulance Service attended.
'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 about 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 about 19:00 BST, followed by a second about 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" about the incident.
Ms Patel later issued a statement condemning the stabbings as a "senseless attack on people simply enjoying a Saturday evening with family and friends".
Labour leader Sir Keir Starmer said the stabbing was "very concerning".
The leader of Reading Council, Jason Brock, said it was "an entirely senseless and horrific attack" in "a very busy area with lots of people socialising".
"Reading is an incredibly strong community, it celebrates its diversity and I would expect that - just as in all trying times - people in Reading will come together and they wont allow themselves to be divided," he told BBC Radio 5 Live.