An American academic has given a graphic account of the moment the London Bridge stabbing attack began, saying it “felt like a warzone”.

Bryonn Bain told the BBC that victim Jack Merritt had been the first person to confront Usman Khan when he launched his knife assault during a prisoner rehabilitation conference on Friday.

“I saw people die, I saw things that I will never be able to unsee,” he said.

Vigils have taken place for Mr Merritt, 25, and second victim Saskia Jones, 23.

Three other people were also injured in the attack before Khan was shot dead by armed officers on London Bridge – two are still in hospital in a stable condition.

Prof Bain said former offenders attending the University of Cambridge-linked conference “stepped up and intervened” to tackle Khan, and people at Fishmongers’ Hall owed their lives to the actions of those who had previously spent time in jail.

He said two men from his performance poetry workshop immediately ran towards shouts from elsewhere in Fishmongers’ Hall in the City of London as the attack began, and as shouts grew louder he also went to assist.

“That’s when I ran down and saw the scene unfolding there,” he said. “I was able to see the attacker.”

He added: “It felt like a warzone… it felt like total chaos.”

Prof Bain said course co-ordinator Mr Merritt was “the first line of defence”.

“I want to honour him,” Prof Bain said of Mr Merritt. “I want to honour his father’s wishes which have been explicit to not have his life be used for political purposes to ramp up draconian policies, because that’s not what he was about.”

Mr Merritt’s father criticised newspaper coverage of Prime Minister Boris Johnson’s pledge to review the early release of convicted terrorists.

Writing in the Guardian, David Merritt says his son “would be seething at his death, and his life, being used to perpetuate an agenda of hate that he gave his everything fighting against”.

The article calls for a justice system that focuses on rehabilitation, rather than revenge, and criticises indeterminate sentences, saying his son worked for “a world where we do not lock up and throw away the key”.

Prof Bain added: “I want to make sure that as much as possible that we uphold the heroes of the day, were formerly incarcerated people, some of the folks who are often easiest to dehumanise.

“They stepped up and many of the folks in that space would not be here today if it weren’t for these guys who did time in prison and literally saved lives.”

In other developments on Monday: 

• Prime Minister Boris Johnson defended his response to the attackafter Mr Merritt’s father criticised newspaper coverage of Mr Johnson’s pledge to review the early release of convicted terrorists
• Mr Johnson and Labour leader Jeremy Corbyn attended a vigil at the Guildhall near London Bridge to honour those caught up in the attack
• London Mayor Sadiq Khan said the best way to defeat the hatred shown in the attack was to focus on the values of hope, unity and love
• BBC News learned the attacker, Usman Khan, 28, had been under investigation by the security service MI5 since his release from prison last year, but given one of the lowest priorities. He had been convicted of a terrorism offence in 2012
• As part of his release conditions, Khan was obliged to take part in the government’s desistance and disengagement programme – which aims to rehabilitate those involved in terrorism

Vigils for the victims of the attack were also held in Cambridge and Anglia Ruskin University, which Ms Jones had previously attended.

Mr Merritt and Ms Jones both studied for masters degrees at the University of Cambridge’s institute of criminology and had been taking part in an event for its Learning Together programme – which focuses on education within the criminal justice system – when they were killed.