Three people were killed and three seriously injured when an assailant went on a stabbing rampage in a park filled with people relaxing in the southern English city of Reading on Saturday, police said.
The Thames Valley Police said they had launched a murder investigation and were not treating the incident as terror-related after making one arrest at the scene.
Prime Minister Boris Johnson condemned the "appalling incident" and other leading politicians expressed sympathies for those hurt at the Forbury Gardens park in the historic centre of the city of 220,000.
"This is not currently being treated as a terrorism incident, however officers are keeping an open mind as to the motivation for the incident and are being supported by colleagues from Counter Terrorism Policing South East," the police said in a statement.
They confirmed arresting a 25-year-old man but made no reference to media reports suggesting he was Libyan.
"My thoughts are with all of those affected by the appalling incident in Reading and my thanks to the emergency services on the scene," Johnson tweeted.
Home Secretary Priti Patel called it "a senseless attack on people enjoying a Saturday evening with friends".
A Black Lives Matter protest took place at the park earlier but police said there was no indication the two incidents were connected.
"In terms of the protest and the people who attended from Black Lives Matter, we're all safe," the Reading BLM event organiser Nieema Hassan said on Facebook.
"None of us are affected. We had all left by the time this happened."
- 'Stabbed in the neck' -
Witnesses reported seeing two air ambulances and several police cars rush to the park on Saturday evening.
One witness described a lone assailant walking through a park filled with people relaxing on the grass and stabbing them at random.
"The park was pretty full. A lot of people sat around drinking with friends," Lawrence Wort told the Press Association.
"One lone person walked though, suddenly shouted some unintelligible words and went around a large group of around 10, trying to stab them."
Wort said he saw three people being stabbed "in the neck and under the arms".
The assailant then lunged at another group and "got one person in the back of the neck", Wort said.
"Then when he realised everyone was starting to run, he ran out the park," said the witness.
Officials said two people were being treated in the emergency department of Reading's Berkshire Hospital.
"An horrific, dreadful incident," policing minister Kit Malthouse tweeted.
The main opposition Labour party's leader Keir Starmer called the incident "very concerning".
Media reports said police conducted a search of the property where the suspect is believed to have lived.
Britain has witnessed two terror-related attacks in the past year.
A convicted jihadist who was out on parole after serving a sentence for terror offences was shot dead by police after stabbing five people -- two fatally -- by London Bridge in the heart of the British capital in November.
Police killed another assailant who injured three people in another London stabbing attack in February.
An overwhelming majority of Britain's serious crimes are committed with knives and other stabbing weapons because of the country's very strict gun ownership laws.
Johnson's Conservative government promised to toughen up penalties for terror-related crimes after winning a sweeping mandate in a December general election.