Protest and privilege make uneasy bedfellows.

It's getting dark in Manhattan, thousands of young people in hoodies have taken to the streets, and brightly-coloured sweets are crunching underfoot.

Seventeen-year-old Trayvon Martin was carrying a bag of skittles, a bottle of iced tea and no weapons when he was shot dead in Florida last month, and the police have still failed to punish his killer. At this march, called in protest at institutional violence against young black men in America, demonstrators from Occupy Wall Street toss skittles over the pavement and carry iced tea, and hundreds of voices chant Trayvon's name.

For the poorest urban communities, police brutality and financial injustice have always been part of the same equation - and it's just the same with the fightback.On the corner of Washington Square, you can hear them yelling in unison: 'We are all Trayvon Martin!'. Teenagers run with their hoods pulled up, stopping traffic as they go. Other names are chanted - the names of Sean Bell, the young man murdered by New York Police the night before his wedding in 2006, and of Ramarley Graham, the unarmed 18 year old shot by police in his home just a month ago. The name of Troy Davis. The name of Martin Luther King. The one name I conspicuously do not hear being chanted is that of Barack Obama. For these young people, the passive politics of hope are distinctly insufficient.

The killing of an unarmed black boy by public officials is not normally an unusual enough occurrence in America to warrant a national outcry. Graham's shooting, which took place in his bathroom in the Bronx in full view of his six-year-old brother, made few front pages.This time, though, in conjunction with the Occupy Wall Street protesters and following a national call for a 'million hoodie march', young people have taken to the streets. They have done so in far greater, angrier numbers than were expected. The police can't do anything about it. There are too many of them, and the crowd is lousy with cameras ready to get a shot of another New York police officer beating another child on a march against police brutality.

They wear hoods, the global uniform of otherness, of the underclass: dependable jumpers that you can hide in to feel safe in a world of surveillance. It's not just for Trayvon Martin, but “for every kid like him,” 29-year-old Laura explains to me as we run down 6th avenue. “I have nephews. Any one of them could have been shot by a George Zimmerman. He wasn't even arrested. We're tired of it.”

When systematic unfairness becomes part of the weave of daily life, it is always hard to determine at what particular moment the heart says enough. It is hard to pinpoint how many bloodied young women, how many dead young men and how many police officers walking casually away it will take before the collective imagination of a community or a country decides that there has been one outrage too many.

We live, however, in interesting times; times when the everyday landscape of injustice is suddenly exposed with all its ugly contours and pitfalls, when what was suffered in silence for years suddenly becomes an occasion for an uprising. As the army of hooded, angry people whoops and hollers and streams down 6th avenue, the expression on the faces of the white couples eating dinner in nearby restaurants is familiar to me. I saw the same mask of quiet panic flicker over the faces of friends and colleagues, peering out of their windows during last summer's riots in London, which were also catalysed by the fatal shooting of a young black man by police. It's an expression that says: who are these kids, and where did they come from? Have we always lived side by side with them and never noticed?

The answers are is yes, and well-to-do-liberals might be advised seriously to consider quite how they will react, however progressive they feel themselves to be, the next time an enormous crowd of angry black teenagers in hoods marches up the high street. Today, significantly, it is impossible to tell who has worn their everyday clothes to this 'hoodie' march, and who has pulled on a hood and tracksuit and come carrying skittles simply in a show of solidarity. I spot university professors in hoods, homeless buskers in hoods, journalists in hoods, and one Hollywood actor in a hood - from a few feet away, they all look the same, a menacing mass with justice and vengeance on its mind.

Perhaps the North American protest movement is about to learn, to its credit, what the British youth uprisings failed at first to anticipate: that one cannot deal with issues of race and class simply by ignoring them. In the months before the August riots, the student and anti-cuts movements had gradually been becoming a little older, a little whiter and a little more middle class, as the schoolchildren from the inner cities who had driven the momentum of the initial protests gradually drifted away. The test of any self-proclaimed people's movement is how it deals with the fact that the people do not all experience oppression in the same way.

During two years of protest reporting, I have watched white, middle-class liberals on both sides of the Atlantic realising with wide-eyed shock, at the first wallop from a police baton, that whoever you are, the state and its agents are to be feared when you step out of line. They are to be feared whatever your colour, even if all you wanted to do was dance around and waggle some placards. The horror that police might treat young white protesters the way they have been treating young black people for years has been with this protest movement from the start. Whilst the killing of young black boys is rarely news in North America, when young white women were kettled and pepper-sprayed by the police during the first days of Occupy Wall Street their tear-smeared faces made headlines across the world.

The learning curve of white privilege is steep, especially for those who weren't aware that they had it. Whereas six months ago, white activists were shouting that police were part of the 99% and encouraging their fellow occupiers to welcome law enforcement into the protest camps, the chant today is: “police are the army of the rich!”

Outrage against systemic police brutality galvanised Occupy Wall Street and united disparate communities under that banner, many black activists are unhappy with what they see as the “co-option” of Trayvon Martin's death. “They're trying to make out that it's the same thing, and it's really, really not,” says 23-year old Nike. Outside Union Square, hundreds of NYPD officers are massing for the nightly head-knocking. “I guarantee you, the first people trampled and arrested will be people of colour,” yells Lee, also 23, as ranks of armed police march into the square, as the barricades go up and the batons come out. “Like every time. I guarantee it.”