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 â€œhelp restore peace and order and to protect the citizens of Fergusonâ€.
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 â€œno alternativeâ€ but to respond.
In a news briefing, Police Captain Ron Johnson said protesters had thrown Molotov cocktails and bottles at security forces, and set up barricades.
â€œFor 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,â€ he told journalists in Ferguson on Monday.
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 private autopsy report found that Mr Brown was 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 â€œas soon as possible.â€