Thousands of demonstrators gathered in several cities across the United States.
They are angry about what they say is racial bias by police.
What has prompted the protests?
Two black men have been shot in confrontations with officers in Louisiana and Minnesota since Tuesday.
Philando Castile died of gunshot wounds after he was pulled over in his car by police in Minnesota.
The scene was filmed by a female passenger in the car.
The US Justice Department is investigating the shooting of Alton Sterling in Louisiana.
“A terrible tragedy has befallen the people of Minnesota, especially the family and friends of Philando Castile,” said Minnesota Governor Mark Dayton.
“I expressed on behalf of all decent-minded Minnesotans, which is just about everyone, our deepest condolences, our shock, our horror.”
Figures for 2015 suggest more than a thousand people are killed by police and law enforcement agencies in the US.
No official federal record is available but the Guardian newspaper keeps its own tally. You can see the results here
The final total for 2015 suggests the death rate for young black men was five times higher than for white men of the same age.