“Counter Terrorism Policing can now confirm that the stabbing incident that happened in Reading last night has now been declared a terrorist incident,” Thames Valley Police said in a statement.

Prime Minister Boris Johnson, who has described the incident as appalling, held a meeting on Sunday with security officials, senior ministers and police to be updated about the investigation, a spokesman for Johnson said. 

Initially police and the government had said the incident did not appear to be terrorism and they were keeping an open mind as to the motive. They said they were not looking for any more suspects in relation to the incident.

The stabbings took place on a sunny evening hours after a rally by anti-racism protesters in the park, Forbury Gardens, but appeared to be unrelated to that..

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“The Black Lives Matter event had finished some three hours previous and was by all accounts a very well conducted peaceful demonstration and this is not a connected attack with that whatsoever,” Jason Brock, the head of the local council authority, told the BBC.

Current coronavirus restrictions mean venues like pubs are closed, so many people in Britain gather in parks in the evenings to meet friends.

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.”

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Police said that “a number of people were injured and taken to hospital. 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.

The incident came hours after a Black Lives Matter demonstration at Forbury Gardens, but police said there was no connection between the attack and the protest.

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 that 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.

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It had previously stood a notch higher, at “substantial” for several years. The country has been hit by a series of violent attacks in recent years, including a suicide bombing at an Ariana Grande concert in Manchester in 2017 that killed 22 people and two deadly vehicle and knife attacks in London the same year.

Airline worker Carlos Garcia Pascual was walking to his home near Forbury Gardens when emergency vehicles and police officers descended. He 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.”

Additional reporting by Associated Press, DPA