The London Bridge attacker said he was “here for the police” during his stabbing rampage, one of the men who fought him has said.

Darryn Frost, a civil servant, had been attending a conference held at Fishmongers’ Hall by Cambridge University’s Learning Together programme alongside Usman Khan on 29 November.

He described hearing shouting downstairs before running out of a room to see someone lying in a pool of blood, and grabbing an ornamental narwhal tusk.

Mr Frost, 38, recalled seeing Khan confronting fellow delegate and former prisoner Steve Gallant with two knives.

“He was slashing at Steve, but Steve had a chair,” Mr Frost told the Guardian. “I pointed the tusk at Khan’s belly and the balance of power shifted in our favour. Khan turned and said to me: ‘I am not here for you. I am here for the police. I am waiting for the police.’”

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Khan, who was wearing a fake suicide vest, allegedly added: “I’ve got a bomb.”

The attack was claimed by Isis but Khan’s ultimate target and motivations have not been made clear.

By the time the exchange took place, the terrorist had already murdered Learning Together coordinator Jack Merritt and volunteer Saskia Jones.

Mr Frost handed Mr Gallant, a convicted murderer who had been jailed in 2005, the narwhal tusk and fled to take the second ornament off the wall before returning to see Khan leaving the building.

He said the terrorist had his knives raised and seemed to be “going for the public, going for maximum damage” as he was chased by delegates including another convict wielding a fire extinguisher.

“He came at me and, instinctively, I stabbed him in the lower abdomen,” Mr Frost said, describing how Khan was pulled to the floor before he jumped on top of him to hold him down.

An armed police officer can be seen pulling him off Khan before opening fire on the attacker at close range.

Mr Frost, who works for HM Prisons and Probation, said he has since suffered survivor’s guilt but has been helped by others who fought the terrorist.

He has started a project called Extinguish Hate to “challenge the spread of fear, hatred or intolerance within our communities”.

Ex-offender who took on London Bridge attacker says he was prepared to lose his life

Inquests are to be held into the deaths of Mr Merritt, 25, and Ms Jones, 23, who both died from stab wounds to the chest.

Like proceedings over the 2017 terror attacks, they will examine potential failings by probation services, the police, security services and other authorities.

Khan was under investigation by MI5, but had been given permission to travel to London for the Learning Together event.

Staffordshire Police is being investigated by a watchdog for its role in the management of Khan following his release from prison.

He had been jailed for his part in an al-Qaeda inspired bombing plot in 2012 and was freed by mistake in December 2018.

Khan’s stabbing, as well as a suspected terror attack at HMP Whitemoor last week, has raised fresh questions over processes for deradicalising terror offenders in prison and assessing the risk they pose.

Four days after the attack at Fishmongers’ Hall, a separation centre designed to hold the most dangerous terrorist prisoners was closed.