LONDON (AFP) - About a thousand Britons protested outside the United States embassy in London on Wednesday in sympathy with demonstrations across the US over the killing of a black teenager by a white police officer.

The decision not to prosecute the officer for shooting dead unarmed 18-year-old Michael Brown in Ferguson, Missouri, sparked anger and racially-charged unrest in cities across the United States this week.

The London protesters held signs reading "black lives matter" and chanted "hands up don't shoot", the slogan adopted by protesters in the US.

Relatives of black men killed by police in Britain addressed the crowd.

"We need to send a message to Mike Brown's family," said Carol Duggan, the aunt of Mark Duggan, a 29-year-old black man whose shooting by police in 2011 was followed by riots.

"We feel the pain, we know the pain, of losing somebody at the hands of the police.

"That is why we stand in solidarity with the community of Ferguson. I feel they are very strong and brave people."

Some protesters carried candles, and a minute of silence was held in honour of those killed by police around the world.

Marcia Rigg, the sister of Sean Rigg, a 40-year-old black musician who died in police custody in London in 2008, told the crowd that she did not support arson and looting but that she understood people's rage.

"People around the world understand the frustration and anger that the people are feeling when our loved ones are murdered on the streets," Rigg said.

Over two thousand National Guard soldiers were deployed to back up the police in Ferguson, a suburb of 21,000 inhabitants outside the city of St Louis, after fires and looting at protests this week.

The outcry followed a conclusion by a grand jury that police officer Darren Wilson acted in self-defence. Wilson has said that he believes he acted appropriately.

Following the vigil at the embassy, hundreds of protesters marched to the central London shopping district of Oxford Street, where they stopped traffic.