A civil servant has described the moment he confronted the London Bridge terrorist with a narwhal tusk as the terrorist threatened to detonate a suicide vest.

Darryn Frost, 38, a South African who has lived in the UK for 14 years and works in the Ministry of Justice, intervened after Usman Khan had fatally wounded Saskia Jones and Jack Merritt at a criminal justice conference at Fishmongers’ Hall on 29 November.

Video footage of the incident showed Frost and other members of the public fending off Khan as they moved on to London Bridge. Frost could be seen pinning Khan to the ground and being pulled away by a police officer before Khan was shot dead.

Facebook Twitter Pinterest Darryn Frost has launched a project called Extinguish Hate to ‘challenge hate and spread kindness’. Photograph: PA

Frost said he had been attending the Cambridge University-run event with colleagues and he ran with others to help when he heard a commotion downstairs.

“I took a narwhal tusk from the wall and used it to defend myself and others from the attacker,” he told the Press Association. “Another man was holding the attacker at bay with a wooden chair. I ran down the stairs, stood next to the man with the chair, and the two of us confronted the attacker.

“He had knives in both hands and, upon seeing me with the narwhal tusk, pointed at his midriff. He turned and spoke to me, then indicated he had an explosive device around his waist. At this point the man next to me threw his chair at the attacker, who then started running towards him with knives raised above his head.”

Frost passed his tusk to the other man and raced back upstairs to rearm with a second tusk from the wall. When he returned, the first tusk was shattered across the floor and the attacker had fled out of the building. Frost and others pursued him on to the bridge.

“We called out to warn the public of the danger and, after a struggle, managed to restrain him to the ground,” he said. “At that point I was trying to isolate the blades by holding his wrists so that he could not hurt anyone or set off the device.”

He thanked those who confronted the attacker and also those “who put themselves in danger to tend to the injured, relying on us to protect them while they cared for others.”

He offered condolences to the families of Jones and Merritt. The two victims had both been involved in campaigning for penal reform, and Frost said: “In reading about their lives and work I am convinced they represent all that is good in the world, and I will always feel the deep hurt of not being able to save them.”

Play Video London Bridge terror attack: vigil held in memory of victims Saskia Jones and Jack Merritt

Both victims were graduates of Cambridge University. Jones, 23, had applied to become a police officer, while Merritt worked for Learning Together, a programme run by the university’s institute of criminology.

Frost said he had omitted “many details” in his account of the incident “out of respect for the victims and their families” and because of an ongoing investigation and inquests.

He said he now wanted “time and space” to continue his recovery from the “traumatic events of that day”, but he urged the media to use its “amazing ability to connect with people” as a force for good to “unify this country”.

“I feel we all have a duty to challenge the spread of fear, hatred or intolerance within our communities,” he said.

He urged people to get behind a project he has launched called Extinguish Hate, to “challenge hate and spread kindness”, and he asked people to donate to fundraisers set up in honour of Jones and Merritt.