Mourners were to hold a minute's silence in the English town of Reading on Monday for the victims of a stabbing that killed three people in the latest attack authorities attributed to terrorism.
Three people were also injured when a man wielding a five-inch knife went on the rampage on Saturday in a park in the southern town, stabbing people at random who were enjoying a sunny, summer evening.
Describing the incident as an atrocity and terrorism, police say they have detained one man and are not hunting others.
A Western security source, speaking on condition of anonymity, told Reuters that the arrested man was a 25-year-old Libyan called Khairi Saadallah. Police have not named the suspect though they confirmed the arrest of a 25-year-old man.
The security source said 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," he added.
When asked about the reports of the assailant being already on authorities' radar, Britain's Security Minister James Brokenshire said the government did not comment on intelligence matters.
Teacher James Furlong, 36, is the only victim to have been named so far. "James was a wonderful man. He was beautiful, intelligent, honest and fun," his parents said in a statement.
The attack was reminiscent of some recent incidents in Britain that authorities also 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.