The families of Michael Brown and Eric Garner were among the demonstrators in the heart of the US capital for the “Justice For All March,” part of a growing protest movement sparked by the fatal August shooting of the unarmed Brown, 18, by a white policeman.

A grand jury decision last month not to indict police officer Darren Wilson over the killing in Ferguson, Missouri was followed by another grand jury decision that also declined to charge another white policeman in the “chokehold” death in Staten Island of father-of-six Garner.

Their deaths, and that of 12-year-old Tamir Rice — shot dead last month by Cleveland police as he brandished a toy gun — unleashed simmering resentment of police tactics in the United States and underlined the distrust many African American men feel towards law enforcement.

A protester holds a sign during the "Justice For All" march in Washington, DC demanding justice for black men who have died at the hands of white police, the latest in weeks of demonstrations across the United States. Picture: AFP Source: AFP

Rice’s family and relatives of Trayvon Martin — shot and killed in Florida by a neighbourhood watchman in 2012 — were also at the march.

Several thousand people massed shouting “No justice, no peace!” -- a signature chant of the nationwide demonstrations — as Garner’s widow took to the stage before the energised crowd, vowing to stand up for all potential victims in honour of her slain husband.

“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,” Esaw Garner said.

Some in the crowd, which was made up of black and white people, held aloft banners proclaiming, “Stop racist police,” “I can’t breathe,” and “President Obama seize this moment. The ancestors are watching.” “I can’t breathe” were the last words uttered by Garner, whom police wrestled to the ground in New York’s Staten Island for allegedly selling untaxed cigarettes.

Justice ... Esaw Garner (R), the wife of Eric Garner, who died while being detained by police, speaks with Reverend Al Sharpton (L) during the "Justice For All" march in Washington, DC. Picture: AFP Source: AFP

Tragic loss ... Eric Garner and wife Esaw Garner. Eric was killed in police custody by a choke hold. Picture: Supplied Source: Supplied

In downtown New York, thousands more gathered despite chilly temperatures, with protesters shouting “We will shut New York City down” and “Black lives matter.” Cole Fox, 24, a bartender, 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.

There was also a small street protest in Boston.

In Washington, Laura Murphy, of the American Civil Liberties Union (ACLU), bellowed: “We demand Congress pass a racial profiling act.” “We are here today, we will be here tomorrow, we will be here until the job is done,” she said to loud cheers from the swelling crowd.

Garner’s mother Gwen Carr echoed that, saying protests would continue until lawmakers responded to demands for justice.

“We will come here as many times as it takes. We will come here over and over and over again,” she told a cheering crowd as they edged toward the Capitol.

“When we go home today, we hope that they have heard our voices, they yield to our commands, because no justice, no peace.” The atmosphere was largely defiant but peaceful, although police stood by in large numbers.

Thousands take part in the Justice for All March and Rally on Pennsylvania Avenue to the US Capitol in Washington, DC, to protest the killings of unarmed African Americans by police officers and the decisions by Grand Juries to not indict them. Picture: AFP Source: AFP

Civil rights activist Al Sharpton, president of the National Action Network and a prominent figure in rallies that have spread in recent weeks beyond Washington and New York, led the protest march.

He told the large crowd that he would press on with protests in honour of those who have died at the hands of police and called for sweeping justice reforms.

“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

Leading the pack ... Lesley McSpadden, mother of police shooting victim Michael Brown helps lead the "Justice For All" rally and march against police brutality and the killing of unarmed black men by police in the nation's capital. Picture: Getty Images/AFP Source: AFP

Lost ... Police shooting victim Michael Brown from Ferguson, Missouri. Picture: Supplied Source: Supplied