Police have clashed with protesters in Missouri, after a grand jury refused to charge a policeman over the death of unarmed black teenager Michael Brown.
Announcing the decision, state prosecutor Robert McCulloch said the jury exhaustively examined evidence.
Mr Brown's family said they were "profoundly disappointed". Shots were fired, and cars and buildings burned.
Michael Brown, 18, was killed by Darren Wilson in Ferguson on 9 August. His death sparked weeks of demonstrations.
Late on Monday, hundreds of protesters gathered outside the police department in Ferguson, a suburb of St Louis.
They followed on radios and mobile phones as Mr McCulloch made a statement explaining the decision.
The prosecutor said that the jury's 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.
Outside the police department, Mr Brown's mother burst into tears as the decision was made public. Some in the crowd began throwing objects at a police line.
Shortly afterwards shots were fired, and protesters were seen vandalising police cars, at least one of which was set on fire. Police responded with smoke and tear gas to try to disperse the crowd.
Footage from the scene showed firefighters tackling a large blaze and heavy black smoke at a retail building, with reports of a pharmacy and a pizza shop also on fire.
Several other buildings have been broken into and looted.
The Federal Aviation Authority said it was restricting the path of some flights into St Louis amid the unrest.
Thousands of people protested around the country, including in Los Angeles, Philadelphia and New York. Protesters in Oakland, California, were blocking traffic on a major motorway.
Mr Brown's family had joined public officials in calling for calm.
After the announcement, they issued a statement saying they were "profoundly disappointed that the killer of our child will not face the consequence of his actions".
President Barack Obama urged Americans to accept "that this decision was the grand jury's to make,'' and called on those who wanted to protest against it to do so peacefully.
The case has stoked racial tensions in the US, with many in the African American community calling for Mr Wilson to be charged with murder.
Protesters have been chanting, "Hands up, don't shoot" - a reference to statements by some witnesses who said Mr Brown had his hands up in apparent surrender to the officer when he was shot.
Police have said there was a struggle between the teenager and the officer before the shooting.
On Monday authorities released more than 1,000 pages of grand jury documents. Among them is testimony by Mr Wilson in which he says that before the shooting, Mr Brown had pushed him back into his car, hit him, and briefly grabbed the gun that Mr Wilson had drawn.
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.
Mr McCulloch said the biggest challenge for his office was the "24-hour news cycle and an insatiable appetite for something - for anything - to talk about".
Ahead of Monday's announcement, Missouri Governor Jay Nixon had declared a state of emergency in the area and called in 400 National Guard troops in anticipation of protests.