US police said early on Tuesday they have arrested 31 people while two have been shot during another night of racially charged protests in Ferguson, Missouri, sparked by the fatal shooting of an unarmed black teenager by a white policeman 10 days ago.
Demonstrations, mostly peaceful but with spasms of violence by smaller groups, have flared since Michael Brown, 18, was shot dead while walking down a residential street on Aug 9.
State Highway Patrol Captain Ron Johnson, briefing reporters on Monday's night's violence, said "our officers came under heavy gunfire" in one area.
"Not a single bullet was fired by officers despite coming under heavy attack," he told a news conference. Riot police had confiscated two guns from protesters and what looked like a petrol bomb. Four officers had been injured.
Johnson separately told CNN that two people were shot within the crowd, but not by police, and were taken to hospital. There was no immediate word on their condition.
The violence has captured headlines around the world, raising questions about the state of U.S. race relations nearly six years after Americans elected their first black president.
"This has to stop. I don't want anybody to get hurt. We have to find a way to stop this," said Johnson, an African-American who grew up in the area and who took over security efforts after the mostly white local force was accused of using excessive force against blacks.
An overnight curfew has been imposed and the National Guard, the US state militia, has been deployed in the St Louis suburb of 21,000 people to stop looting and burning that have punctuated the protests.
President Barack Obama and civil rights leaders have appealed for calm while a federal investigation into the shooting proceeds.
"While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns, and even attacking the police, only serves to raise tensions and stir chaos," Obama told a news conference on Monday.
"It undermines, rather than advancing, justice."
Bottles hurled
The Monday night's clashes between riot police and protesters followed hours of mostly peaceful demonstrations, Reuters witnesses said.
Police had closed a roadway to traffic to provide a path for marches but said a smaller group within the larger crowd hurled bottles, rocks and petrol bombs at officers standing near armored vehicles. Police responded by firing gas-filled canisters and a noise cannon to try to disperse the throng.
Some demonstrators, including a church minister using a blow horn, urged crowds to calm down.
There have been largely peaceful protests over Brown's killing elsewhere in the United States including in St Louis, New York, Seattle and Oakland. Police commander Johnson said some of those arrested had come from California and New York.
Missouri Governor Jay Nixon declared a state of emergency in Ferguson on Saturday and a curfew from midnight to 5am He also mobilised the National Guard to back up state police.
Obama said he told the governor the use of the National Guard should be limited and called for conciliation in communities hit by the unrest.
US Attorney General Eric Holder will visit Ferguson on Wednesday, Obama said.
Holder said over 40 FBI agents were canvassing Ferguson neighborhoods in their investigation and an additional medical examination was being performed on Brown. Results of autopsies done by federal and St Louis County authorities were pending.