Thousands of people have marched through the US capital, Washington DC, to protest against the recent killings of unarmed black people by police.
Relatives of Michael Brown, shot dead in the Missouri town of Ferguson, and Eric Garner, who died being restrained in New York, were among them.
Both died after encountering police, but grand juries decided not to bring charges, sparking anger and unrest.
A demonstration in New York also drew thousands despite chilly weather.
Speakers at the Capitol called for changes to US legislation.
Lesley McSpadden, mother of Michael Brown, told the crowd: "What a sea of people. If they don't see this and make a change, then I don't know what we got to do. Thank you for having my back."
The mood in the US capital was described as calm but defiant, with a large number of police on standby.
Earlier in the day, a small group of protesters from Missouri disrupted the schedule by taking to the stage at the starting-point, on Freedom Plaza, and blowing a bullhorn.
They complained that the protest, which was organised by long-established civil rights groups, was staid and ineffective.
At the scene: Rajini Vaidyanathan, BBC News, Washington DC
The brisk winter weather did not deter the thousands of protesters. Their aim - to stop what they say are the unlawful killings of black men, at the hands of the police.
The majority of people I spoke to said it was the case of Eric Garner, who died after being held in a chokehold by police in New York, which had motivated them to take to the streets, many for the first time. That case, as well as the fatal shooting of the unarmed black teenager Michael Brown in Ferguson, has galvanised calls for change.
People say they will continue to protest until they get justice. They key question is what does that justice look like? For some that means changing legislation and the grand jury system, or giving police body cameras. But the bigger challenge is changing mindsets.
Michael Brown, 18, was shot dead on 9 August during an altercation with a white police officer in Ferguson.
Mr Garner, 43, died while being held down by a white police officer on 17 July.
He had been challenged over the alleged sale of loose cigarettes on a street in Staten Island, New York.
The event was caught on camera and his dying plea of "I can't breathe" has become a slogan of the protesters. It echoes the adoption of "Hands up! Don't Shoot!" - a Ferguson refrain alleging that Mr Brown was surrendering to police when the fatal shots were fired.
Relatives of three other black people killed in controversial shootings were also expected to attend the march, according to the National Action Network:
- Akai Gurley, 28, was shot dead by New York police on 20 November
- Tamir Rice, 12, was shot dead in a Cleveland, Ohio, park on 22 November while carrying a pellet gun
- Trayvon Martin, 17, was shot dead on 26 February 2012 by a neighbourhood watch volunteer in Sanford, Florida
Marchers crowded Pennsylvania Avenue for the walk from Freedom Plaza to the Capitol, but the actual numbers were not immediately clear.
Some in the crowd, which was made up of both black and white people, held banners saying: "Stop racist police", "I can't breathe", and "President Obama seize this moment. The ancestors are watching."
Speaking at the Capitol, Mr Garner's mother, Gwen Carr, called the march a "history-making moment".
"It's just so overwhelming to see all who have come to stand with us today," she said. "I mean, look at the masses. Black, white, all races, all religions... We need to stand like this at all times."
Most people who spoke to the BBC's Rajini Vaidyanathan at the march cited Mr Garner's death as the main factor which had led them to join the protest, she said in a tweet.
The Rev Al Sharpton, a leading civil rights advocate, called for "legislative action that will shift things both on the books and in the streets".
In New York, protesters shouted "We will shut New York City down" and "Black lives matter", an AFP news agency correspondent reports.
New York march co-organiser Umaara Elliott urged "action... at every level of government to ensure that these racist killings by the police cease".
The decision not to prosecute a policeman over Michael Brown's death sparked riots in Ferguson and as far away as Oakland, California.
However, most of the protests over his and Mr Garner's death have been peaceful.