A 25-year-old man was detained at the scene and has been arrested on suspicion of killing three people, police say.
British police have said they were treating a stabbing spree in which a lone assailant killed three people in a park filled with families and friends in the southern English city of Reading as a "terrorism incident".
The Thames Valley police said three people were also seriously wounded and a 25-year-old resident of the historic town 60 kilometres 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.
Britain’s national news agency, Press Association and other media outlets named him as Khairi Saadallah, a Libyan asylum-seeker living in Reading.
A Reading man of that name who is the same age as the suspect was sentenced to two months in prison last year for assaulting an emergency worker. The same man was also charged last year with assaulting a judge who had sentenced him.
The BBC reported that Saadallah was investigated by British security services last year over concerns he planned to travel abroad to join a militant group, but that he was not determined to be a major threat.
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."
PM Johnson 'appalled'
Prime Minister Boris Johnson said he was "appalled and sickened" by the bloodshed at Forbury Gardens –– a picturesque 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.
Wide ranging police cordon in #reading as reports of multiple fatalities after nearly a dozen stabbed at site of BLM/Antifa Marxist rally. pic.twitter.com/HEoMW2iVOu— Jack Dawkins (@DawkinsReturns) June 20, 2020
Violence on sunny evening
The violence erupted around 1900 GMT as families and friends were enjoying a warm, sunny evening in the Forbury Gardens park in Reading, a town of about 200,000 residents 64 kilometres west of London.
Witnesses reported that police cars and helicopters descended on the park.
Within minutes police had blocked off several roads, and two air ambulances landed nearby.
Witness recalls stabbings
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, severely in the neck, and under the arms, and then turned and started running towards me, and we turned and started running," Wort said.
"When he realised that he couldn’t catch us, he tried to stab another group sat down. 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."
My deepest condolences to the families mourning loved ones this morning. 🙏🏾 and thoughts and prayers are with all those who witnessed #Readingattack— I Can't Breathe...Dawn Butler MP (@DawnButlerBrent) June 21, 2020
Terror alert level
A Black Lives Matter protest took place at the park earlier but police said there was no indication the two were linked.
"None of us are affected," the Reading BLM event organiser Nieema Hassan said on Facebook.
"We had all left by the time this happened."
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.