The US town of Ferguson has seen rioting and looting after a jury decided not to bring charges over the killing of a black teenager.
Michael Brown was shot by a white police officer, Darren Wilson, on 9 August, sparking protests.
A police chief said the latest violence in the suburb of St Louis, Missouri, was "probably much worse" than on any night since the teenager's death.
St Louis County police chief Jon Belmar said rioters had fired 150 shots.
Many in the African-American community had called for Mr Wilson to be charged with murder, but after three months of deliberation a Missouri grand jury - of nine white and three black members - made no recommendation of charges.
President Barack Obama joined the teenager's family on Monday in appealing for calm, urging Americans to accept the decision was "the grand jury's to make''.
At the scene: Joanna Jolly, BBC News, Ferguson
The sun's shining this morning on South Florissant, which saw some of the most violent demonstrations outside the Ferguson Police Department last night.
Local residents have been up since the early hours cleaning up the streets. Shopkeepers are boarding up shops. A small group of protesters is yelling at half a dozen police standing outside the department.
A group of residents is standing outside a beauty parlour which was looted last night. Its windows have been smashed in and they're hoping to stop anyone else coming in and looting.
"We're trying to come together and get past this", says Judy. Everyone's expecting more demonstrations tonight.
"They let our town burn," says Anastasia Knowles. "They sacrificed us for Clayton," she says referring to the choice to deploy the state national guard there and not in Ferguson.
Authorities said more than 80 people were arrested amid chaos in several areas of St Louis overnight. Sixty-one of those arrests were in Ferguson, with charges including burglary and trespassing.
The fabric of the community, Mr Belmar said, had been "torn apart" in Ferguson, which is a predominantly black community patrolled by a mainly white police force.
BBC correspondent answers your questions
As protesters charged barricades, hurling glass bottles, police responded with smoke and tear gas.
One protester, Charles Miller, told the BBC that while he did not advocate violence, he understood why people were angry.
"You can't just go shoot an 18-year-old who's unarmed on the street, despite what the story may have been," he said.
Thousands of people also protested in other US cities, from Los Angeles to New York.
In Oakland, California, they blocked traffic on a major highway in the San Francisco Bay area.
Officer Darren Wilson's testimony
Mr Wilson said he tried to block Mr Brown and another man in the street with his police vehicle in connection with a robbery, but when he tried to open the car door, Mr Brown slammed it shut.
The police officer said he managed to reopen the door, pushing Mr Brown back with it, and then the teenager hit him in the face. In the struggle which followed, Mr Wilson said, the teenager tried to grab his drawn gun while insulting him.
Mr Wilson said he fired several shots during the struggle before Mr Brown ran off. When Mr Brown stopped running, the officer said, he ordered him to get on the ground but Mr Brown advanced on him instead, putting his right hand under his shirt in the waistband of his trousers. Mr Wilson said he then fired the fatal shots.
Read more of Darren Wilson's testimony (Warning: Explicit language)
Much of the debate since August has centred on whether Michael Brown was attempting to surrender to Darren Wilson when he was shot, and protesters have adopted the chant "Hands up, don't shoot".
But state prosecutor Robert McCulloch, speaking after the grand jury decision, said physical evidence had contradicted some of the witness statements.
Police say there was a struggle between the teenager and the officer before the shooting.
Mr Wilson himself says that before the shooting, Mr Brown had pushed him back into his car, hit him and grabbed at his drawn gun.
The jury was made up of 12 randomly picked citizens from the state of Missouri. At least nine votes were needed in order to issue an indictment.
Mr Brown's family said in a statement: "We are profoundly disappointed that the killer of our child will not face the consequence of his actions."
But they also appealed for calm, saying: "Let's not just make noise, let's make a difference", and calling for all police to wear body cameras.
Mr Brown's mother, Lesley McSpadden, wept at news of the jury's decision as she was comforted by supporters outside the police station in Ferguson.
Mr Brown's family could yet file a wrongful-death lawsuit against Mr Wilson.
Meanwhile, a justice department investigation is still under way into whether the police officer violated Mr Brown's civil rights.
Darren Wilson, 28, is currently on paid leave and has kept out of the public eye.
The department is also investigating practices at the Ferguson police department.