Police have declared the stabbing attack in Reading in which three people died a “terrorist incident”.
A 25-year-old man from Reading was arrested at the scene of the attack at about 19:00 BST on Saturday, which also left three people seriously injured.
Metropolitan Police Assistant Commissioner Neil Basu, head of counter-terrorism policing, said: “This was an atrocity.”
He said police have found “nothing to suggest anyone else was involved”.
Security sources told the BBC the man arrested is thought to be Libyan.
Mr Basu said police were working with the coroner to formally identify those who had died and he praised the actions of unarmed Thames Valley Police officers, who detained the suspect.
He also praised members of the public who helped in the emergency and said 41 witnesses had so far come forward.
He said “the motivation for this horrific act is far from certain” but added it was clear that it was not associated with an earlier peaceful Black Lives Matter demonstration at Forbury Gardens, the site of the attack.
Thames Valley Police Chief Constable John Campbell said it was a “truly tragic incident”.
“Incidents of this nature are very rare, though I know that will be of little comfort to those involved and understand the concern that this incident will have caused amongst our local community,” he said.
Speaking on the BBC’s Andrew Marr Show, Health Secretary Matt Hancock said Home Secretary Priti Patel and Prime Minister Boris Johnson would be in contact with police leading the investigation later on Sunday.
“It’s clearly a terrible crime whatever the motivation and we will be making sure the police have all the support they need,” he said.
The prime minister met security officials, police and senior ministers at Downing Street on Sunday at 09:00 BST and was updated on the ongoing investigation.
An eyewitness to the attack at Forbury Gardens, a park near the centre of Reading, said he saw a man moving between groups of people in the park, trying to stab them.
Three people died in the attack, and another three were seriously injured. Two of the injured people have been discharged and one remains in hospital, although the injuries are not thought to be life-threatening.
The arrested man had previously been in prison in the UK for a relatively minor conviction, not a terrorism offence, sources told the BBC.
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”.
A block of flats on Basingstoke Road in Reading was raided by more than a dozen armed police officers carrying shields at about 23:00 BST on Saturday.
About an hour and a half after they entered, a loud bang was heard at the scene, after which several of the armed officers left.
Craig O’Leary, chairman of the Thames Valley Police Federation, said the “swift action” of officers at the scene prevented “potentially more lives from being lost”.
“There are barely words to describe their bravery – officers who ran towards danger with the sole thought of protecting the public we serve,” he said.
One officer “rugby tackled” the suspect to the ground, according to a report in the Sunday Mirror.
Floral tributes to the victims have been left at the scene. One read: “”There are no words that anyone can say to express how horrible and senseless this was.
“Our prayers are with all the victims and their families and friends.”