A stabbing rampage that killed three people and wounded three others in the southern English town of Reading was an act of terrorism, police say, calling the attack in a sunny park an atrocity.
A 25-year-old man was arrested on suspicion of murder after the stabbings on Saturday evening in Reading, about 65km west of London..
A Western security source, speaking on condition of anonymity, told Reuters that the man, who remains in police custody, is a Libyan called Khairi Saadallah.
Britain's most senior counter-terrorism police officer, Neil Basu, said a man had run into Forbury Gardens and attacked people with a knife before being detained by unarmed officers.
He said it had been declared terrorism after detectives worked through the night on the case.
"This was an atrocity," Basu said.
He said there was nothing to suggest anyone else was involved and they were not hunting further suspects.
While the motivation for the attack was far from certain, he said there was no intelligence that crowded places were at risk.
The victims have not yet been identified.
Prime Minister Boris Johnson held a meeting on Sunday with security officials, senior ministers and police to be updated about the investigation.
He said he was "appalled and sickened" by the attack and said Britain would change the law if needed to prevent any future incidents.
"If there are lessons we need to learn ... we will learn those lessons and we will not hesitate to take action if necessary," he said.
Initially police and the government had said the incident did not appear to be terrorism and they were keeping an open mind as to the motive.
A witness said the attack began when a man suddenly veered toward a group of about eight to 10 friends and began stabbing them. The attack took place hours after a rally by anti-racism protesters in the park but Basu said the two incidents were not related.
"The Black Lives Matter event had finished some three hours previous and was by all accounts a very well conducted peaceful demonstration and this is not a connected attack with that whatsoever," Jason Brock, the head of the local council authority, told BBC TV.
The nature of the attack was reminiscent of a number of recent incidents in Britain that authorities considered to be terrorism.
In February, police shot dead a man, previously jailed for promoting violent Islamist material, who had stabbed two people on a busy street in south London. Last November another man who had been jailed for terrorism offences stabbed two people to death on London Bridge before he too was shot dead by police.