British police said Sunday they were treating a stabbing spree in which a lone assailant killed three people in a park filled with families and friends in Reading as "terrorism".
The police said three people were also seriously injured and a 25-year-old resident of the historic town 60 kilometres (35 miles) west of London was detained.
Local and counter-terror police made no reference to reports that the man was a Libyan refugee who has been granted asylum in Britain.
Footage showed what appeared to be a large number of counter-terror police performing a controlled explosion at the suspect's presumed residence shortly after the Saturday evening attack.
"This morning, I can formally confirm that this has been declared a terrorist incident," counter-terror police chief Neil Basu said.
"Although the motivation for this heinous act is far from certain, Counter Terrorism Policing South East has taken on responsibility for leading this investigation."
Prime Minister Boris Johnson said he was "appalled and sickened" by the bloodshed at Forbury Gardens -- a popular park situated next to the ruins of the 12th-century Reading Abbey.
"If there are lessons that we need to learn about how we handle such cases, about how we handle events leading up to such cases, then we will learn those lessons, and we will not hesitate to take action when necessary," Johnson said in televised comments.
A Black Lives Matter protest took place at the park earlier but police said there was no indication the two were linked.
"We had all left by the time this happened," the Reading BLM event organiser Nieema Hassan said.
Britain's terror alert level was kept at "substantial" -- the third highest on a five-point scale.
"There is no specific intelligence to suggest anyone else attending crowded places is at risk," the counter-terror police chief said.
- 'Shouted unintelligible words' -
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".
A local school cancelled Monday's classes after its history teacher was identified as one of the dead.
Another witness told The Sunday Mirror that one group of people who were attacked were "just in a circle chatting".
A third person who witnessed the arrest said the bloodied suspect looked dazed.
"He was just staring," witness Amir Hadyoon told reporters.
"He was just completely silent. He wasn't even blinking. He looked really weird."
Paramedic teams were seen trying to resuscitate the injured as they lay unresponsive on the ground.
"An horrific, dreadful incident," policing minister Kit Malthouse tweeted.
The main opposition Labour party's leader Keir Starmer said Britons were "united in their grief".
"This is not a time for party politics," the opposition leader said.
Britain has witnessed two terror-related attacks in the past year.
A convicted jihadist 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 a London stabbing attack in February.
An overwhelming majority of Britain's serious crimes are committed with knives and other stabbing weapons because of 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.
Well-wishers have left floral tributes outside a local police station in tribute to the victims of the attack
Counter terrorism police have taken over the investigation in to the stabbing in Reading, England
The attack happened at a popular park situated next to the ruins of the 12th-century Reading Abbey
One witness described a lone assailant walking through a park filled with people and stabbing them at random
Map locating a fatal knife attack which killed three people Saturday evening in Reading, west of London. A suspected has been detained by police.
Two air ambulances and several police cars rushed to the park on Saturday evening