The English town of Reading was to hold a minute's silence on Monday for the victims of a stabbing that killed three people including an American in the latest attack linked to terrorism.
Three people were also injured when a man wielding a five-inch knife went on the rampage on Saturday in a park, stabbing people at random who were enjoying a sunny, summer evening.
A Western security source, speaking on condition of anonymity, told Reuters that a 25-year-old Libyan suspect called Khairi Saadallah had been arrested.
The Philadelphia Inquirer reported one of the dead was U.S. citizen Joe Ritchie-Bennett, 39, who had lived in Britain for 15 years.
"I offer my deepest condolences to the families of those killed in the attack on June 20," U.S. ambassador to London Woody Johnson said on Twitter.
"To our great sorrow, this includes an American citizen."
Teacher James Furlong, 36, who was friends with Ritchie-Bennett according to media reports, was also killed. "He was beautiful, intelligent, honest and fun," his parents said.
The third victim has not yet been identified.
Describing the incident as terrorism, police say they have detained a 25-year-old man and are not hunting others.
The security source told Reuters that Saadallah had come across the radar of Britain's domestic security agency MI5 last year over intelligence he had aspirations to travel for extremist purposes, although his plans then came to nothing.
Reading, which is about 40 miles (65 km) west of London, said a minute's silence would be held at 0900 GMT.
"What you appear to have here is a lone actor and they are obviously particularly hard to detect," said Jonathan Hall, the Independent Reviewer of Terrorism Legislation, whose job is to inform public debate on security laws.
"If, as has been reported, the individual is suffering from poor mental health that is a particularly difficult area."
The attack was reminiscent of some recent incidents in Britain that authorities also called 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.