A court on Tuesday convicted two men of the 1993 murder of black teenager Stephen Lawrence, a landmark case that exposed racism in the London police and led to a change in the law allowing suspects to be tried twice for the same crime.

The 18-year-old student was stabbed to death at a bus stop in southeast London in an unprovoked attack by a gang of white youths shouting racist abuse.

Gary Dobson, 36, and David Norris, 35, were found guilty after a six-week trial that hinged on new scientific evidence presented by prosecutors.

Lawrence's mother Doreen and father Neville wept as the verdicts were delivered at the Old Bailey, having spent years waging a high-profile campaign to get his killers brought to justice after police botched the initial investigation.

Dobson protested his innocence as he was led from court saying: You have condemned an innocent man here, I hope you can live with yourselves.

Doreen Lawrence said she felt a mixture of anger and relief. How can I celebrate when my son lies buried? ... These verdicts will not bring my son back, she told reporters outside court.

Stephen Lawrence is buried in Jamaica, from where his parents emigrated to Britain in the 1960s.

Had the police done their job properly, I would have spent the last 18 years grieving for my son rather than fighting to get his killers caught, she added. The fact is, that racism and racist attacks are still happening in this country.

INSTITUTIONAL RACISM

The case became a catalyst for change after exposing deep-rooted failings in London's Metropolitan Police, dominated by senior white officers in an increasingly multiracial society.

A 1999 report by senior judge William Macpherson said the murder had exposed institutional racism in the force and also accused officers of incompetence and a failure of leadership.

Since then, the police have overhauled their policies on racism and tried to recruit more officers from ethnic minorities, but the Lawrence case still weighs heavily on the force.

Prime Minister David Cameron praised the Lawrence family for their tireless fight for justice.

Today's verdict cannot ease the pain of losing a son. But, for Doreen and Neville Lawrence, I hope that it brings at least some comfort after their years of struggle.

Opposition Labour leader Ed Miliband said the case had been a wake up call for Britain.

I am proud that 18 years on from Stephen's murder, Britain is a much more tolerant and open country, he said.

We have new laws to prevent the stirring of racial hatred, while following the Macpherson Report, reforms were made to ensure that our public institutions are better representative and responsive to all the people, he said.

But we must never allow ourselves to become complacent about the threat of racism. And we must continue to confront it, in all its forms.

The Lawrence case also helped end the judicial doctrine of double jeopardy, which had previously prevented suspects from being tried twice for the same crime.

One of the defendants, Dobson, had been acquitted of the murder in 1996 when a private prosecution brought by the teenager's parents collapsed.

The Court of Appeal quashed that acquittal in May 2011 and said Dobson could stand trial again, a decision made possible after double jeopardy was scrapped in 2005.

The trial of Dobson and Norris, which began in November, hinged on new forensic evidence linking the two men to the murdered teenager.

Prosecutors said textile fibres, blood and hair belonging to Lawrence had been found on clothing seized from the defendants. The defence argued the clothes were contaminated during the investigation because officers did not store them properly.

The two men will be sentenced on Wednesday. They face a mandatory life term, with a minimum number of years to be set by the judge before they can be considered for release.

(Reporting by Keith Weir and Estelle Shirbon)