Leanne O'Brien wept and clutched a cuddly toy as she was supported by family and friends at the event, which also honoured fellow Cambridge graduate Saskia Jones.

Merritt, 25, and Jones, 23, were both stabbed by 28-year-old convicted terrorist Usman Khan during a prisoner rehabilitation event they were supporting in London on Friday.

The Cambridge vigil took place as Prime Minister Boris Johnson and Opposition Leader Jeremy Corbyn stood side-by-side to pay their respects at a separate event at Guildhall Yard in London, observing a minute's silence alongside members of the public.

The pair were joined by Mayor of London Sadiq Khan, who called for people to come together following the killings and work for a future "not defined by hatred but defined by hope, unity and love".

"We come together this morning as Londoners to remember, to honour and to mourn the innocent lives lost as a result of this horrific terrorist attack on Friday."

Khan added: "The best way to defeat this hatred is not by turning on one another but by focusing on the values that bind us."

The mayor thanked the public and the emergency services who "ran towards danger, risking their lives to help others".

Usman Khan was wearing an electronic monitoring tag when he launched the attack, which injured three others, after he was invited to the prisoner rehabilitation conference on Friday.

The event was organised by Learning Together, a program associated with Cambridge University's Institute of Criminology.

The attack has prompted the Ministry of Justice to review the licence conditions of every convicted terrorist released from prison, which Prime Minister Boris Johnson said was "probably about 74" people.

Johnson has vowed to take steps to ensure people are not released early when they commit serious offences.

But the family of Merritt asked that his death not be used to justify introducing "even more draconian sentences" on offenders in a heartfelt tribute released on Sunday.

They said: "Jack lived his principles; he believed in redemption and rehabilitation, not revenge, and he always took the side of the underdog.

"We know Jack would not want this terrible, isolated incident to be used as a pretext by the government for introducing even more draconian sentences on prisoners, or for detaining people in prison for longer than necessary."

In a tweet on Sunday, Merritt's father David said: "Don't use my son's death, and his and his colleague's photos - to promote your vile propaganda.

"Jack stood against everything you stand for - hatred, division, ignorance."

Jones, a volunteer with Learning Together, was described as having a "great passion" for providing support to victims of crime by her family.

In a statement, they said: "Saskia had a great passion for providing invaluable support to victims of criminal injustice, which led her to the point of recently applying for the police graduate recruitment program, wishing to specialise in victim support."

Khan was convicted of terror offences in February 2012 and released from prison on licence in December 2018, halfway through his 16-year prison sentence.

On Friday he launched his attack armed with two knives and wearing a fake suicide vest.

He was tackled by members of the public, including ex-offenders from the conference, before he was shot dead by police.

Australian Associated Press