Black teen-ager Stephen Lawrence was stabbed to death at a London bus stop by a gang of white youths who shouted racist abuse and swallowed him up, prosecutors said Tuesday.

The attack on 18-year-old Lawrence in April 1993 is seen as a pivotal episode in Britain's race relations.

Lawrence, who had hoped to become an architect, was targeted simply because of the colour of his skin in an unprovoked assault, the Old Bailey court heard.

Gary Dobson, 36, and David Norris, 35, from London, deny murdering Lawrence. The trial is expected to run until late December or January.

Dobson was acquitted of Lawrence's murder in 1996 after a private prosecution brought by the Lawrence family collapsed.

An Appeal Court this year ordered a retrial after quashing Dobson's acquittal on the basis of new evidence submitted by prosecutors.

The case sparked a public outcry after police failed to prosecute the suspected killers and a damning report later accused officers of incompetence and institutional racism.

Prosecutor Mark Ellison said the group of white men acted as one, surrounding Lawrence and forcing him to the ground, the Press Association reported. His friend Duwayne Brooks managed to run away, shouting get up and run, Steve.

One of the group hurled a racist insult and at the same time the whole group rushed towards them, Ellison told the court. One witness described that (Lawrence) was swallowed up by the weight of numbers and forced to the ground, he added.

They reacted together as one on seeing two black men. The only discernible reason for the attack was the colour of his skin.

Despite being stabbed twice, Lawrence managed to get up and run for 220 metres (yards) before collapsing on the pavement. He died later in hospital.

Ellison said the case against Norris and Dobson relied on new scientific evidence which emerged after a review of the case in 2007.

He told the jury of eight men and four women that a grey jacket and a multi-coloured cardigan are among items of clothing which form the basis of the new evidence.

It consists of the finding of textile fibres, blood and hair linked to Stephen Lawrence on clothing seized from the defendants, Ellison told the court.

The defence argues that the samples got on to the clothing through contamination during the police inquiry, he added.

Judge Colman Treacy has described the trial as an important and sensitive case that has already been the subject of another court hearing, an inquest and public inquiry.

(Reporting by Peter Griffiths)