Gordon Brown joined Doreen Lawrence at the private service
The mother of Stephen Lawrence has said her pain remains "undiminished" as she marked 15 years since his death in a racist attack.
Doreen Lawrence was joined by more than 300 people including Prime Minister Gordon Brown at a memorial service in central London.
Stephen, 18, was murdered by white youths in Eltham, south-east London in 1993 but no-one was convicted.
Justice Secretary Jack Straw said his death left Britain a "nation shamed".
The Archbishop of Canterbury Dr Rowan Williams led the event at St Martin-in-the-Fields Church in Trafalgar Square.
Opposition leader David Cameron and Liberal Democrat leader Nick Clegg also attended the service.
'Make a difference'
Dr Williams paid tribute to the Lawrence family for turning their suffering into "hope, planning, care and vision".
Stephen's brother Stuart Lawrence urged the congregation to continue to work to "make a difference" in Stephen's name.
"I hope that one day justice will be served in the name of my brother one way or another," he said.
"It only takes one man, one woman, one child, one mother, father, uncle, sister, cousin or one brother to make a difference in life."
Stephen was attacked by a gang of five white men at a bus stop
Mr Straw said: "It should not have taken the outrageous murder of a thoroughly innocent young man, killed for the sole reason that his skin was black and not white, to be the catalyst for change which should have happened years and years before that."
In February, the Stephen Lawrence Centre in Deptford, south-east London, was attacked by vandals a week after the educational facility was opened.
Mr Straw added: "The despicable vandalism of the Stephen Lawrence Centre earlier this year is another symbol of the sickness in some people's minds and how far we still have to go."
Before the one-hour service, Mrs Lawrence said: "For me and Stephen's family, the pain of our loss remains undiminished after 15 years."
Mrs Lawrence and her former husband Neville campaigned to expose police failings in the case.
She said: "The campaign for justice for Stephen continues as must the wider fight to combat racism, discrimination and prejudice."
An inquest into the teenager's death found he was "unlawfully killed in a completely unprovoked racist attack by five white youths".
Gary Dobson, Neil Acourt, Luke Knight, Jamie Acourt, and David Norris were arrested over Stephen's murder but the Crown Prosecution Service failed to prosecute anyone due to insufficient evidence in 1994 and again in 2004.
In November, Metropolitan Police said they were reinvestigating new forensic evidence and the men could face another trial.
Stephen's killing contributed to the abolition of the double jeopardy law two years ago, which prevented a person being tried twice for the same crime.