Thousands of people have demonstrated in major cities along the US East Coast demanding an end to what they say is police brutality.
The protests on Wednesday came after a young African-American man died of injuries sustained in custody in Baltimore last week.
The biggest show of people power was in Baltimore itself, where several thousand mostly young demonstrators paralysed several city blocks in a major rally through downtown to City Hall.
Thousands more protested in New York, Boston and the capital Washington DC in solidarity.
Protests largely peaceful
The protests were overwhelmingly peaceful and good-natured, although New York police detained several demonstrators and emotions were running high.
What appears to be a growing movement for change was centred on Baltimore, where a rally that started at the main train station included black and white demonstrators, some of them linking arms and chanting: "No justice, no peace! No racists, no peace!"
Many in the march, which appeared to be gathering momentum, were high school or college students.
"We're protesting the ongoing injustices that police have perpetrated on black men particularly. Police are trigger-happy and we need to stop that," Jonathan Brown, 19, a student at Johns Hopkins University, told AFP news agency.
And he warned: "If you can't get your way through non-violent means because the aggressor is violent, other action needs to be taken."
Some in the huge crowd held placards, one reading, "Killer cops deserve cell blocks." A few wore shirts with the words, "Amnesty International observer".
RELATED: Community activists help calm Baltimore unrest
The 2,000 National Guard personnel who have flooded Baltimore this week kept a low profile, although authorities have said they are primed to swoop should the march spark unrest like that which flared following the funeral of Freddie Gray, 25, on Monday.
Gray's death was the latest instance in the US of a black man succumbing at the hands of police - a situation that has stirred resentment among African Americans who believe they are targeted by police.
That anger also sparked coast-to-coast demonstrations in major US cities last year after a white policeman shot dead an unarmed black teenager in the St Louis suburb of Ferguson in August.
Baltimore authorities urged calm and warned that they would again enforce a citywide curfew beginning at 10pm (02:00 GMT on Thursday) and lasting until 5am.
Tuesday night's curfew was largely respected, although police made 35 arrests during the night and warned that they were assessing the volatile situation minute-by-minute.
Maryland Governor Larry Hogan said he had been "very encouraged" by the last 24 hours and said a semblance of normality was returning to Baltimore, a gritty city of 620,000 about an hour's drive from Washington DC.
But he cautioned: "We are not out of the woods yet." And he warned demonstrators to respect the curfew: "There are peaceful protests happening tonight. We want to make sure individuals can exercise their First Amendment rights and express their concerns.
"We also want to stress and remind everyone that there is a 10pm curfew in place in the city and I urge everyone in Baltimore to get off the streets at 10pm."
New York arrests
In New York, protesters gathered at Union Square, in Lower Manhattan, for a rally dubbed on a Facebook page, "NYC Rise up and Shut it down with Baltimore."
"We call on New Yorkers from across the five boroughs, #BlackLivesMatter activists and organisations as well as all organisations that stand for social, economic, and racial justice to rally," said the organisers.
The march initially met no resistance from police, but that swiftly changed as officers moved in and made arrests.
The New York Police Department would not say how many were detained, though CNN put the number at a couple of dozen.
Smaller demonstrations were held in Boston and Washington DC, US media said.