Officers in riot gear shot rubber bullets and tear gas to disperse protesters ahead of the Sunday curfew
The US state of Missouri is sending the National Guard to the town of Ferguson as protests escalate over the police shooting of an unarmed black teenager.
Governor Jay Nixon signed an order to "help restore peace and order and to protect the citizens of Ferguson".
The decision was made as police clashed with angry crowds shortly before a second night under curfew began.
Police in Ferguson, a suburb of St Louis, said they came under attack and had "no alternative" but to respond.
Cpt Ron Johnson said protesters had thrown Molotov cocktails and bottles at security forces, and set up barricades before the five-hour curfew began at midnight (05:00 GMT).
"For those who would claim that the curfew was what led to [the] violence, I will remind you this incident began three and a half hours before the curfew was to have started," he told journalists in Ferguson on Monday.
What is the National Guard?
Protesters ignored Sunday's curfew and marched down the city's main streets
Tensions between the demonstrators and police quickly escalated
Security forces accused some of the demonstrators of advancing on a police command post
The killing of Michael Brown by a white policeman in a street on 9 August has inflamed racial tensions in the largely black suburb.
A preliminary autopsy by the St Louis County Medical Examiner's office on the day after Mr Brown's death found he had been shot, police said, without disclosing how many times.
But a preliminary private autopsy report found that he had been shot at least six times, including twice in the head, the New York Times reports.
US Attorney General Eric Holder has ordered a federal post-mortem on the body of the 18-year-old, to take place "as soon as possible".
The bullets did not appear to have been shot from very close range due to the lack of gunpowder on the victim's body, forensic pathologist Michael Baden was quoted by the paper as saying.
He flew to Missouri on Sunday to conduct a separate autopsy at the request of the family.