The St Louis suburb of Ferguson has seen rioting and looting after a jury decision not to bring charges over the killing of Michael Brown.
A local police chief said the violence was “probably much worse than the worst night we ever had in August” after the black teenager was killed.
St Louis county police chief Jon Belmar said he had heard 150 shots fired by crowds.
Brown was shot by a white police officer on 9 August, sparking protests.
Many in the African American community had called for police officer Darren Wilson to be charged with murder.
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.”
Most of at least a dozen burned businesses were “total losses”, he said, and two police cars had been “basically melted”.
“I don’t think we were underprepared,” he added. “But I’ll be honest with you, unless we bring 10,000 policemen in here, I don’t think we can prevent folks who really are intent on destroying a community.”
Some protesters charged barricades and taunted police. Chants of “murderer” could be heard.
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.
“He was unarmed and you are an armed law enforcement officer who’s been trained in combat. So I think people are rightfully upset.”
After the situation calmed down, he added, he hoped there would be an “opportunity to really grow and change a lot of things, and make sure [Michael Brown’s] death didn’t mean nothing”.
Explaining the jurors’ decision, state prosecutor Robert McCulloch said their job had been to separate fact from fiction, and that some witness statements had been contradicted by physical evidence.
“These grand jurors poured their hearts and soul into this process,” he said.
Protesters have been chanting, “Hands up, don’t shoot” – a reference to statements by some witnesses who said Mr Brown had had his hands up in apparent surrender to the officer when he was shot.
Police say there was a struggle between the teenager and the officer before the shooting.
In his own testimony, Wilson says that before the shooting Mr Brown pushed him back into his car, hit him and briefly grabbed his drawn gun.
The jury was made up of 12 randomly picked citizens – nine white and three black.
At least nine votes were needed in order to issue an indictment.