Thousands of protesters paralyzed parts of New York and Washington on Saturday, stepping up demonstrations across the United States demanding justice for black men killed by white police.

The rallies in the US capital, New York, Boston and in several Californian cities were among the largest in a growing protest movement sparked by the killing of unarmed black teenager Michael Brown in Ferguson, Missouri on August 9.

Grand jury decisions not to prosecute the white officers responsible for 18-year-old Brown's death and a fatal chokehold on New York father of six Eric Garner in July, have triggered weeks of protests.

Demonstrators shut down parts of Manhattan and Washington's Pennsylvania Avenue that leads to the Capitol with cries of "No justice, no peace!" "Justice Now!" and "The whole damn system is guilty as hell!"

The mixed crowds of black and white, mobilized many young people but also young families, parents and the elderly. They held up banners proclaiming "Stop racist police" and "I can't breathe."

"I can't breathe" were the last words uttered repeatedly by Garner, as police wrestled him to the ground for allegedly selling untaxed cigarettes in New York's Staten Island.

Their deaths lit a touchpaper on simmering resentment against police tactics in the United States and distrust many black men feel toward law enforcement.

The Garner and Brown families were joined in Washington by relatives of 12-year-old Tamir Rice, who was shot dead last month by Cleveland police, and of Trayvon Martin, who was killed in Florida by a neighborhood watchman in 2012.

History-making moment

Garner's widow and wife took to the stage before the energized crowd.

"I am here not only for marching for Eric Garner, but for everyone's daughters and sons and nieces and nephews and dads and moms," widow Esaw Garner said.

Garner's mother Gwen Carr said the protests would continue until lawmakers respond to demands for reform.

"This is a history-making moment," she said as onlookers erupted in cheers.

"We will come here as many times as it takes," she told a cheering crowd as they edged toward the US Capitol building that houses Congress.

Civil rights activist Al Sharpton, president of the National Action Network and a prominent figure in the rallies, led the protest march in Washington.

PHOTO GALLERY: MILLIONS MARCH IN NY JUSTICE RALLY

Organizers called for lawmakers to "pass a national profiling act" and Sharpton called for sweeping justice reform.

"You thought it would be kept quiet. You thought you'd sweep it under the rug. You thought there would be no limelight. But we're going to keep the light on Michael Brown, on Eric Garner, on Tamir Rice, on all of these victims," he thundered, as the families of those killed joined him on stage, some sobbing.

In New York, protesters shut down a four-mile (six-kilometer) route from Washington Square, down Fifth and Sixth Avenues and Broadway to converge outside police headquarters, filling the air with chants of "Justice now!"

'Shut New York City down'

Christmas shoppers stood agog on the sidewalk, supporters hung out of apartment windows to shout words of encouragement and store workers took a break to take pictures on their cell phones.

"We will shut New York City down," promised the organizers into loudspeakers as the crowd streamed out of Washington Square.

Members of the throng carried black cutouts of human figures bearing the words "RIP," along with the names of those killed by police, while others shouted "Justice now! The whole damn system is guilty as hell."

Bartender Cole Fox, 24, marched with his mother and held a banner reading "Grand Jury Reform Now."

"Fundamental changes need to be made. It's just a matter of days before the next person, black or white, is killed," he said.

Student Rosalind Watson, 21, decried "institutional racism."

"If one person sees this march and feels more supported and safe, it'll be a success," she told AFP at the spirited protest.

Police and organizers said thousands turned out in New York, but were not immediately able to provide a clearer breakdown.

The atmosphere was largely defiant but peaceful, although police stood by in large numbers.

In Boston, Massachusetts State Police said several people were arrested and some roads blocked in Boston.

In Berkeley, California, an effigy of a black man hung by a noose was placed outside a university entrance with the words "I can't breathe" scrawled on its chest.