On a New York street, 17-year-old Marcel Hamer is handcuffed and lying on his back. There is a cop standing over him with a boot on his chest.

"You wanna get f---ed up?" says the cop, who has stopped Hamer on suspicion of smoking a joint.

"Mister, it was just a cigarette, sir," Hamer says. "It was just a cigarette." The cop bends down and punches him in the face with a thick thwack and the boy goes limp as onlookers shout, "You knocked him out! You knocked him out." The cop sees the camera but does not seem to care. "Get it on film," he says to the crowd.

It could have been worse for Hamer. Other recent footage shows police arresting 43-year-old Erick Garner whom they believe has illegally sold a single cigarette. He is choked to death as they subdue him.

In another New York neighbourhood, 16-year-old Kahreem Tribble, suspected of possession of marijuana, runs briefly from police and then gives up. He stands on a sidewalk with hands raised, caught on security footage. A cop approaches with his gun drawn and thumps him in the face with the weapon. The boy slumps to the ground as other officers surround him. He is punched again. He will later appear in court with cracked teeth.

On another day in another neighbourhood, New York's finest are annoyed that a street fair has run half an hour over time. They wrestle a street vendor to the ground and an officer is filmed kicking him while others hold him down.

The recent spate of viral police violence videos is not limited to New York, though.

By the side of a Los Angeles freeway a grandmother, Marlene Pinnock, is straddled by a traffic cop who pounds her face time and again with a gloved fist. In the isles of a Wal-Mart store in Ohio John Crawford III is browsing. He selects a bb gun from a shelf and minutes later is shot dead by cop.

Footage of police violence against black citizens has threaded itself through social media and onto mainstream news broadcasts and front pages.

In the background, constantly, has been the humming anger on the streets of Ferguson on the outskirts of St Louis, Missouri, where in August a white officer shot dead an unarmed black teenager whom he had stopped for jay walking.

The shooting of Michael Brown provoked protests, riots and a tough - occasionally provocative - police response. As a grand jury considers whether to charge the officer who shot him, Darren Johnson, the street action continues. The demonstrators believe Johnson should have been arrested immediately, and they suspect they are witnessing a cover-up take shape.

Dr Maria Haberfeld, a criminal justice expert from the City University of New York, says that despite the ugly footage online, policing in the United States is less violent and more accountable now than ever before. She goes as far as calling the past 10 years, "the decade of accountability" in policing.

She says the apparent increase in police violence is a product of the ubiquity of phone cameras on the streets and the fact that the footage they capture is available for endless replaying online. She believes we are in the midst of a moral panic over police violence rather than a surge in it.

That said, she believes the violence that does exist is the product of poor training and poor pay creating police forces that do not enjoy the respect of their communities.

Others believe America's police forces have a serious problem with the way they treat minorities, particularly African Americans.

"Unfortunately, the patterns that we've been seeing recently are consistent: The police don't show as much care when they are handling incidents that involve young black men and women, and so they do shoot and kill," Delores Jones-Brown, a law professor and director of the Centre on Race, Crime, and Statistics, also at the City University of New York, told Mother Jones magazine earlier this year.  "And then for whatever reason, juries and prosecutor's offices are much less likely to indict or convict."

She says evidence suggests that police are more likely to shoot black people not only in poor inner city areas, but African Americans in any occupation in any community.

A 2007 report, ColorLines, found that in 10 major cities African Americans were disproportionately likely to be shot by police. Mother Jones reports that in Oakland California the National Association for the Advancement of Colored People found that out of 45 officer shootings between 2004 and 2008, 37 victims were black, none was white.

There is little nationwide statistical evidence to provide a full picture of the trends, though unsurprisingly African Americans are far less likely to trust police than whites.

In a Gallop survey published in July last year, 24 per cent of black men aged 18 to 34 reported they had been treated unfairly by police in the month leading up to the survey. In another Gallop study, published earlier this year, 59 per cent of whites had a high or very high confidence in police compared with just 39 per cent of black people.

Whatever the figures are on actual violence, Dr Haberfeld says the lack of confidence is a danger in itself. "Police forces are a basic pillar of a functioning democracy," she said.

Speaking of his own experiences growing up, Ta-Nehisi Coates wrote in The Atlantic recently that often black people did not call police for help, and even when they did they did so warily.

"We do so fully understanding that we are introducing an element that is unaccountable to us. We introduce the police into our communities, the way you might introduce a predator into the food chain."

Coates believes that the police are not the problem, rather they are the tip of a sword wielded against black America that includes Stand Your Ground laws, the drug war and mass incarceration.

These are forces that are beginning to make themselves felt on the streets of Ferguson as the community waits to hear whether the officer who killed Michael Williams will be charged.

As America looks on it thinks back to that first infamous amateur footage of a mob of white cops beating a black man, the assault of Rodney King in Los Angeles in 1991.

The riots that followed cost 53 lives and $US1 billion ($1.15 billion) in damages, and they began not when the footage went public but when the officers involved were acquitted.

The Ferguson grand jury is expected to come to a decision by year's end.