Neville Lawrence said he struggles to put into words the devastation caused to his family when his son was killed.
Stephen, 18, was murdered by a gang of racists in Eltham, southeast London, on 22 April 1993.
Mr Lawrence said: “The fact that I had to lose my first child has been devastating. I can’t begin to explain the pain and the anguish me and my family have suffered over the past 25 years.”
The 76-year-old has embraced Christianity and he plans to spend the 25th anniversary of his son’s death in church.
Up to six thugs attacked Stephen and his friend Duwayne Brooks.
Two have been convicted of murder, but the rest have evaded justice.
David Norris and Gary Dobson are serving life sentences. Three other men who have consistently been accused of the killing but never convicted are Jamie Acourt, 41, from Bexley; his brother Neil Acourt, 42, who uses his mother’s maiden name Stuart, and Luke Knight, 41, both from Eltham.
The initial investigation into Stephen’s death was dogged by incompetence, racism and alleged corruption.
Mr Lawrence and his ex-wife Doreen met Nelson Mandela two weeks after Stephen died.
He said: “When I met him for the first time I was so inspired by his persona and the way he talked to people.
“He made it clear to us that in his country it was something that they go through every day, but never in his wildest dreams did he think that something like that would happen in a place like Britain.
“Meeting him gave me the courage to do some of the things I have done over the years.
“Other families came to my rescue as well. When you are going to go on a journey, if somebody else who has been through it comes and talks to you they can give you an idea what you’re going to face down the road.
“What those families did for me I can’t even start to explain to people. I decided, after a certain amount of time, on my journey, that if anybody who had the same kind of experience wanted me to come and talk to them then I would do that.
“I also decided that I would go into schools and universities and talk to the younger generation.”
Parts of the UK, especially London, have seen a surge in violent crime this year – with nearly 60 murders in the capital since the start of 2018.
Mr Lawrence speaks to young people about the perils of carrying weapons and said: “Right now with the violence, and the knife crime violence, it is even more urgent now that I talk to these youngsters and explain to them the pain and the suffering they inflict on families.
“It is a life sentence and something that will never be served. I’ve been serving a life sentence for the last 25 years and I will go on serving that until the day I die.”
If young people are left with nothing to do they will get involved in activities that are “devastating” to their community, Mr Lawrence warned.
He and his former wife, who is now Baroness Lawrence, have campaigned for more than 20 years to get justice for their son.
The bungled case led to a major public inquiry and eventually a change in the law to allow Dobson to be tried twice for murder.
The Met has admitted the investigation is unlikely to progress without new information.
But focus on the case will continue with an inquiry into undercover policing, probing claims that police moles infiltrated campaign groups supporting the Lawrence family.
Mr Lawrence’s solicitor Jocelyn Cockburn said: “I am humbled by his message of forgiveness to mark the anniversary of his son’s death.
“Neville can feel proud of what he has achieved in the intervening years.”
Mr Lawrence believes that, in death, his aspiring architect son has become a “legend”.
He said: “When these boys killed my son Stephen, they created a legend. In his death, Stephen is a legend.
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“There is debate about racism, there are organisations set up to help to make people understand about racism, the police have been put under the spotlight because of Stephen’s death.”
:: Neville Lawrence will be live on Sky News at 8.15am on Monday