Riders from around the country were told that “Harry will not be forgotten” as they came together on the third anniversary of the death of the 19-year-old, who was killed when his motorbike was in collision with a car outside the US military base RAF Croughton in Northamptonshire.
The alleged hit-and-run driver, US citizen Anne Sacoolas, was charged with causing death by dangerous driving four months later.
She had diplomatic immunity asserted on her behalf following the crash and was able to leave the country, sparking an international outcry.
At Saturday’s event in London, organiser Steven Baird turned towards the US embassy and told the community of less than 100 bikers who had gathered in front of it: “People who come to our country should obey our laws. They can not and should not get out of our country while disobeying our laws.”
He later said that US President Joe Biden, who has suffered personal tragedy and bereavement, should be aware of the trauma that Mr Dunn’s family are enduring and their need for someone to be held accountable for what happened.
Mr Baird added: “We say that it is not acceptable today that something like that can happen in our country – someone can claim diplomatic immunity and leave.
“The buck stops with Biden. He is the leader. We are just asking for someone to be accountable.
Days before Christmas in 1972, Mr Biden’s 30-year-old wife Neilia and their 13-month-old daughter Naomi were killed in a car accident.
Mr Dunn’s friend Chelsea Wilson, 24, said she was glad to be among those marking the teenager’s anniversary to help ensure her friend, who “liked anything noisy, but he loved his bikes”, is “not forgotten”.
She said: “This would be right up his alley.
“It is important to just remember him. We need to keep the momentum up. We are not going to go away. We are very much still here and supporting the cause.
“What happened was a tragedy. It is important that we are here.
“We are here remembering our friend. He is not going to be forgotten.”
Sacoolas was due to appear at London’s Westminster Magistrates’ Court in January.
However, days before her appearance, the Crown Prosecution Service said the hearing had been vacated to enable ongoing discussions between the CPS and Sacoolas’s legal representatives to continue.
Mr Dunn’s family are marking the third anniversary of his death in private.
Family spokesman Radd Seiger said: “It is a terribly sad day but no sadder than any other day that his family have to endure.
“We all miss him terribly. He was a giant of a man with an even bigger and warmer personality who cared so much about others.
“Harry’s family have chosen to mark the day privately and will not be making any further public comment at this time. They look forward to doing so however as soon as they can.
“In the meantime, and on their behalf, from the bottom of all our broken hearts, I would like to thank the tens of millions of people in the UK, USA and right around the world who continue to support them through their nightmare.
“We continue to be overwhelmed daily by messages of love and support from all over the globe and each and every one honestly gives them the strength to carry on.”