Heavy automatic gunfire has been heard in Ferguson following a decision not to charge a white police officer who shot dead an unarmed black teenager.
St Louis County Police Department said in a tweet that shots had been fired in one part of the city. In another, it said an officer had been shot and a manhunt was underway.
Protesters have torched buildings and police cars after a Missouri grand jury decided Officer Darren Wilson should not be indicted for the death of Michael Brown.
A picture of the officer, taken after the incident, has just been released The 18-year-old was unarmed when he was killed on 9 August.
Petrol bombs, bottles and rocks have been thrown at officers, while Sky News' Joey Jones said he had witnessed looting. Officers in armoured cars responded by firing tear gas to try to disperse the demonstrators.
President Barack Obama urged both protesters and police to show "restraint".
"We need to accept that this decision was the grand jury's to make," he said.
"There are Americans who agree with it and there are Americans who are deeply disappointed, even angry. It's an understandable reaction."
The victim's family said they were "profoundly disappointed that the killer of our child would not face the consequences of his actions" but added that "answering violence with violence is not the appropriate reaction".
Several churches in Ferguson have opened their doors to provide safe haven for members of the public.
A tweet by St Louis County Police said: "A U. City police officer has been shot. Condition is unknown. Search for suspect underway. It is unclear if related to #FergusonDecision".
Peaceful protests are taking place in other US cities such as New York, Washington, Chicago, Los Angeles and Seattle.