The murder of Stephen Lawrence sparked "monumental change" in British society, Prime Minister David Cameron said ahead of a memorial service today to mark the 20th anniversary of the teenager's death.
Stephen, 18, was murdered by a gang of racists who stabbed him as he waited for a bus in Eltham, south-east London on April 22, 1993.
The police investigation into his death was marred by incompetence and allegations of racism, and it took 19 years to bring any of his killers to justice.
Cameron praised the tireless efforts of Stephen's family in their campaign for justice, but acknowledged "more still needs to be done".
The Prime Minister said: "The senseless killing of Stephen Lawrence in 1993 was a tragedy. It was also a moment that sparked monumental change in our society - change that has been brought about by the tireless efforts of Stephen's family in challenging the police, Government and society to examine themselves and ask difficult questions.
"I believe that many of those questions have been answered: from improved community relations to more accountability in policing. Much has been achieved, but we know that more still needs to be done. We owe this to the memory of Stephen."
Stephen's mother Doreen will be joined by friends and relatives at the memorial service which is taking place at St Martin-in-the-Fields church near Trafalgar Square.
A number of high-profile supporters of the charitable trust that she set up in her son's name are also expected to attend, and public figures including Metropolitan Police Commissioner Sir Bernard Hogan-Howe.
He and his predecessors have attended previous memorial events for Stephen, despite the troubled background of the case.
A poster has been put up in New Scotland Yard featuring a personal message from Sir Bernard about the investigation.
It reads: "Twenty years ago the Lawrence family lost their loved son, Stephen. We let them down by not catching his murderers. Then last year we finally brought two of his killers to justice. The Met won't forget Stephen Lawrence."
Stephen's father Neville has chosen to remember his son privately in Jamaica today, taking flowers to his grave and saying a prayer with a friend who is a pastor.
Neville and Doreen Lawrence were forced to battle for nearly two decades for justice for their son, and they finally saw Gary Dobson and David Norris jailed for his murder in January last year.
The aspiring architect was set upon by a gang of five or six thugs, but the remainder of the killers have never been punished.
Mrs Lawrence today said she fears police will wind up their investigation and allow the other members of the gang to remain at large.
"Just having two of them caught will never be enough," she told the Daily Mirror.
"If there were five within the group that killed Stephen, all of them need to be behind bars."
The detective leading the investigation insisted that the inquiry is still live and that his team will follow any leads that come up.
Detective Chief Inspector Clive Driscoll said: "It's a live investigation. All avenues of the investigation will be left open and we will revisit them whenever we feel we have to. You never close your mind to anything.
"We will endeavour to follow all the leads that we can."