Cambridge University graduates Mr Merritt, 25, and Saskia Jones, 23, were both stabbed to death by 28-year-old convicted terrorist Usman Khan during a prisoner rehabilitation event they were supporting in London on Friday.
American academic Bryonn Bain told the BBC that Mr Merritt was the first person to confront the knifeman, adding: "I saw people die, I saw things that I will never be able to unsee."
Khan was released from prison on licence in December 2018, halfway through a 16-year prison sentence after he was convicted of terror offences in February 2012.
The attack 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.
Mr Johnson has also vowed to take steps to ensure people are not released early when they commit serious offences.
The family of Mr Merritt, from Cottenham, Cambridgeshire, asked for his death to not to be used to justify introducing "even more draconian sentences" on offenders in a heartfelt tribute released on Sunday.
And in a tweet on Sunday evening referring to coverage of the attack, his 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."
Writing for the Guardian on Monday, he said his son would be "livid" if he could comment on his death.
"He would be seething at his death, and his life, being used to perpetuate an agenda of hate that he gave his everything fighting against," he said.
"We should never forget that. What Jack would want from this is for all of us to walk through the door he has booted down, in his black Doc Martens.
"That door opens up a world where we do not lock up and throw away the key.
"Where we do not give indeterminate sentences, or convict people on joint enterprise.
"Where we do not slash prison budgets, and where we focus on rehabilitation not revenge.
"Where we do not consistently undermine our public services, the lifeline of our nation.
"Jack believed in the inherent goodness of humanity, and felt a deep social responsibility to protect that. Through us all, Jack marches on."
His father's comments came after Mr Merritt's girlfriend broke down in tears as she attended a vigil in his memory in Cambridge earlier on Monday.
Leanne O'Brien wept and clutched a cuddly toy as she was supported by family and friends at the event, which also honoured Miss Jones.
The Cambridge vigil took place as Boris Johnson and 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.
They 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".
The vigils took place as West Midlands Police said a 34-year-old man arrested in Stoke-on-Trent on suspicion of preparation of terrorist acts had been recalled to prison due to a breach of his licence conditions.
He has been named in reports as Nazam Hussain, who was jailed with Usman in 2012 for terrorism offences, and like Usman had been released early on licence after successfully appealing against his original indeterminate sentence.
Officers from the West Midlands Counter Terrorism Unit held him after a search of his home on Saturday.
The force has said there is no information to suggest he was involved in Khan's attack at London Bridge.
Officers have been searching a home in Lindley Street, Stoke, on Monday, as part of the West Midlands Counter Terrorism investigation.
A resident, who did not wish to be named, told the PA news agency: "I saw the man who lived there on the first day he arrived.
"He's been there two months. He had a prison bag with him on the day he arrived and he met with two CID, so I knew it was something serious.
"The police have certainly been there since yesterday, possibly before."
Khan, from Stoke, was on licence and 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 afternoon.
The knifeman, who was living in Stafford, was given permission to travel into the heart of London by police and the Probation Service.
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.
One of the three people injured in the attack has been allowed to return home while the other two remain in a stable condition in hospital.The event was organised by Learning Together, a programme associated with Cambridge University's Institute of Criminology.
Mr Merritt's father said his son "devoted his energy" to the programme, adding: "He lived and breathed fire in his pursuit of a better world for all humanity, particularly those most in need."
Miss Jones, a volunteer with Learning Together from Stratford-upon-Avon, Warwickshire, was described as having a "great passion" for providing support to victims of crime by her family.
In a statement they said: "She was intent on living life to the full and had a wonderful thirst for knowledge, enabling her to be the best she could be.
"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 programme, wishing to specialise in victim support."
London Bridge was reopened on Monday, while a police cordon remains in place on the west footpath near Fishmongers' Hall.
No-one else is being sought over the attack.