LONDON (AFP) - Prime Minister David Cameron attended a memorial service on Monday marking 20 years since the murder of black teenager Stephen Lawrence, a landmark moment in British race relations.

The premier joined Lawrence's mother Doreen at a church service for the 18-year-old, who was brutally stabbed to death on April 22, 1993, in an unprovoked attack at a London bus stop.

The killing sparked an overhaul of British policing after an official report found that "institutional racism" had tainted the original investigation.

"The senseless killing of Stephen Lawrence in 1993 was a tragedy," Cameron said ahead of the service at St Martin-in-the-Fields church on London's Trafalgar Square.

"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."

Two white men, Gary Dobson and David Norris, were convicted of the murder in January 2012 on the basis of new forensic evidence. They lost an appeal against their life sentences in August.

They were among five suspects arrested within days of Lawrence's murder, and police said on Monday that the investigation into possible accomplices remained "live".

"All avenues of the investigation will be left open," said Detective Chief Inspector Clive Driscoll.

A poster put up at Scotland Yard's headquarters on Monday said that the force had let the Lawrence family down by not catching his murderers immediately.

"Then last year we finally brought two of his killers to justice," it read.

"The Met won't forget Stephen Lawrence." Scotland Yard chief Bernard Hogan-Howe was among the guests at the memorial service, along with interior minister Theresa May, Deputy Prime Minister Nick Clegg and London mayor Boris Johnson.

Soul singer Beverley Knight gave a powerful rendition of "Fallen Soldier", a song she wrote for Lawrence.