A 25-year-old Libyan asylum seeker, Khairi Saadallah, who had previously been in prison for a relatively minor offence, carried out a frenzied knife attack on Saturday evening in a crowded park in Reading, killing three people and injuring three others before being rugby tackled to the ground by a police officer.

The incident appeared not to be connected with a “Black Lives Matter” demonstration that had taken place earlier in Forbury Gardens in the centre of the Berkshire city, because most of the protesters had disbanded by the time of the knife attack at 7pm.

The attacker set upon groups of friends who were simply sitting around relaxing and chatting among themselves.

One eyewitness, Lawrence Wort, 20, a personal trainer, said: “The park was pretty full. One group of friends was sitting drinking when one lone person walked through, suddenly shouted some unintelligible words and went around, 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.

“When he realised that he couldn’t catch us, he tried to stab another group sitting down. He got one person in the back of the neck and then, when he realised everyone was starting to run, he started to try and run out of the park.”

A second witness, Dan, said he thought people were playing “duck duck goose”, a children’s game which sees participants sit in a circle while someone taps them on the head. “He (the attacker) was tapping them on the head. Then I thought someone had been sick. But it was blood spraying out.”

A third witness reported: “A lone sergeant, who responded when the alarm was raised, ran to the scene, saw the attacker running away with a knife and rugby tackled him to the ground.”

Two air ambulances landed in the park, which has now been completely cordoned off.

Gruesome footage on social media revealed the attack’s appalling aftermath — three men lying a few feet apart on blood-soaked grass. “Police arrived within minutes and frantically tried to keep them alive.”

By 11pm more than a dozen armed officers in full armour raided the perpetrator’s top flat in a block in Basingstoke Road in Reading, where a loud bang was heard 90 minutes after police entered.

“Swift action” of officers at the scene prevented “potentially more lives from being lost”, police said, suggesting bomb-making equipment or other lethal devices had been made safe. The entire corridor where Saadallah lived has been sealed off by the police.

While Saadallah remains in custody, Britain’s most senior counter-terrorism officer, Neil Basu of Scotland Yard, has taken personal command of what has now officially been declared “a terrorist incident”.

“This was an atrocity, my deepest sympathies go out to the families who will be mourning loved ones today after this horrific act and to everyone who has been affected by it,” Basu said on Sunday.

“We’re working with the coroner to formally identify all those who’ve died and to inform and support their relatives. I’d like to praise the actions of Thames Valley Police colleagues (in Reading) who responded immediately and they detained the attacker nearby, unarmed and incredibly brave.”

He revealed 41 people who were in the park had so far offered eyewitness accounts to the police. He also said “the motivation for this horrific act is far from certain” but added it was clear that it was not associated with an earlier peaceful Black Lives Matter demonstration.

Prime Minister Boris Johnson, who held a meeting on Sunday with security officials and senior ministers, said he was “appalled and sickened”.

There is now an attempt by the media to put together a profile of Saadallah.

One of his neighbours said: “I’ve only spoken to him for 30 minutes, I didn’t know the guy but it makes me scared. I have a son. I had no idea he could do something like this.”