According to Jon Belmar, chief of the St Louis County Police Department, officers ''had an encounter'' with Mr Brown, during which the youth pushed an officer and reached for his gun.

''It is our understanding at this point in the investigation that there was a struggle over the officer’s weapon. There was at least one shot fired in the car.''

But Mr Brown was found dead more than 10 metres from the car and, according to his mother, he was shot eight times.

Other witnesses have challenged the police account. One witness, Piaget Crenshaw, a young local woman, told a local Fox station that Mr Brown was walking with a friend, Dorin Johnson, to his grandmother’s apartment when a policeman in a car ordered them to get off the road. After a verbal altercation, the officer shot Mr Brown dead as Mr Johnson hid behind a car, she said.

Another has told reporters that Mr Brown had his hands in the air when he was shot.

Mr Brown had graduated from high school earlier this year and was set to start classes at a technical college on Monday.

After the shooting, events followed a tragically familiar path. Soon the streets had filled with protesters, some yelling ''kill the police''. Fearing a riot, police flooded the streets with officers.

On Sunday many of those who attended a vigil for Mr Brown knelt before police with their hands in the air, saying ''don’t shoot'' in protest at the killing.

Some time that night the riot which police had feared broke out. According to reports, a dozen or so businesses were vandalised or looted, two officers were injured and 32 people arrested.

On Monday the streets were again calm but tension remains high, especially as an autopsy reports is soon to be made public. Much of the protest over Mr Brown’s death is now taking place online.

Within hours of the shootings, a photo of Mr Brown’s stepfather had gone viral. He is holding a cardboard sign on which the words ''Ferguson police just executed my unarmed son'' have been written.

And another tragic Twitter meme has erupted since the shooting under the hashtag #iftheygunnedmedown. Angered that media appear to select photos of young black men in the news that make them look menacing, thousands have posted dual photos of Mr Brown or of themselves.

#IfTheyGunnedMeDown confronts how minority deaths are portrayed in the media. http://t.co/WNphllgJGE pic.twitter.com/aLPE8h09HP — Mashable (@mashable) August 12, 2014

One post shows an image of Mr Brown looking tough and streetwise beside another of him looking comfortable in a classroom. Another typical post shows photo of a young man throwing a gang sign beside another of him in military uniform reading to young children.

This is a rhetorical question. And that makes me sad. #iftheygunnedmedown which picture would they use? pic.twitter.com/86UCAa4QHE — Steve Marmel (@Marmel) August 10, 2014

Daily Feminist Cheat Sheet: #IfTheyGunnedMeDown what picture would they use pic.twitter.com/4qcz6rwosc — I Am ... http://t.co/sMw4OLV4B6 — Feministing (@feministing) August 11, 2014

The meme appears to draw parallels between Mr Brown’s shooting and the death of Trayvon Martin in 2012.

In that case, the young black teenager was shot dead by George Zimmerman, a neighbourhood watch volunteer who believed that the teenager, wearing a hooded sweatshirt, looked suspicious.

The St Louis Post-Dispatch reported that Mr Brown's family has retained Trayvon Martin's lawyer, Benjamin Crump, to represent them.

The shooting also comes as civil rights organisations prepare for protests in New York over the death of Eric Garner. Mr Garner, a 43-year-old, died after police allegedly used an illegal ''chokehold'' on him while arresting him for illegally selling single cigarettes on the street.