A stabbing attack in a park in south-east England, in which three people were killed, was described on Sunday by police as an act of terrorism.
The authorities initially said the attack was not being treated as terrorism but later changed their opinion.
Thames Valley Police said they launched a murder investigation.
They said they arrested a man, 25, at the scene and were not pursuing any other suspects.
The arrested man was a Libyan citizen identified by British media as Khairi Saadallah.
"This was an atrocity," said Neil Basu, Britain's most senior counter-terrorism police officer.
"From our inquiries undertaken so far, officers have found nothing to suggest that there were any other people involved in the attack, and we are not looking for anyone else in relation to this incident."
Only one of the three victims, teacher James Furlong, has so far been publicly identified.
Two colleagues, Anne Kennedy and Katie Pearce, said Mr Furlong "truly inspired everyone he taught through his passion for his subject and his dedication".
The violence began about 7pm local time as families and groups were in the Forbury Gardens park in Reading, a town of about 200,000 residents, 65 kilometres west of London.
Personal trainer Lawrence Wort said the park was full of groups socialising on the grass when “one lone person walked through, suddenly shouted some unintelligible words and went around a large group of around 10 trying to stab them”.
“He stabbed three of them in the neck and under the arms, and then turned and started running towards me, and we turned and started running,” Mr Wort said.
“When he realised that he couldn’t catch us, he tried to stab another group.
"He got one person in the back of the neck and then when he realised everyone was starting to run, he ran out the park.”
Witnesses said police cars and helicopters descended on the park after the attack as police blocked off roads in the city centre and two air ambulances were called to treat the wounded.
"A number of people were injured and taken to hospital," police said.
"Tragically, three of these people died and another three sustained serious injuries."
The Royal Berkshire Hospital in Reading said it was treating two casualties from the incident.
Airline worker Carlos Garcia Pascual was walking to his home near Forbury Gardens when emergency vehicles and police officers arrived.
Mr Pascual said it was chaos as police yelled at people to leave the area.
“We didn’t know if it was a situation like happened in London a few years ago, where the attackers were on the loose,” he said.
“Forbury Gardens is a peaceful place. A lot of families go there with their kids to play, picnics.
"To realise that happened in Forbury Gardens is really hard to believe.”
Prime Minister Boris Johnson tweeted that he was "appalled and sickened" by the attack.
Police said they did not think the attack was linked to a Black Lives Matter demonstration in the park hours before the incident.
Nieema Hassan, one of the organisers of Saturday’s protest, said demonstrators had left by the time the violence occurred.
In a social media post, she said she was “praying for the people who are affected. I hope they’re OK”.
Britain’s official terrorism threat level stands at “substantial", the middle level on a five-rung scale, meaning an attack is likely.
It stood a notch higher at “severe” for several years.
Britain has been hit by violent attacks in recent years, including a suicide bombing at an Ariana Grande concert in Manchester in 2017, which killed 22 people, and two deadly vehicle and knife attacks in London the same year.
– Additional reporting by AP