One person has been shot and seven people arrested with police battling to impose a curfew in the riot-hit US town of Ferguson a week after the racially-charged shooting of an unarmed black teenager.

Police in the strife-torn Missouri community said on Sunday the shooting victim was in critical condition after being felled by an unidentified attacker.

The incident came as police used smoke bombs and tear gar to disperse around 200 demonstrators who had defied a curfew imposed by Governor Jay Nixon.

Violence has flared since the shooting death of Michael Brown on August 9 by a white police officer which has renewed a national debate about law enforcement and African Americans.

Police in Ferguson also drew criticism for facing down protesters in military-grade armoured trucks while brandishing high-powered weapons.

Nixon told CNN on Sunday tensions in Ferguson were likely to remain high, citing the community response as “raw and appropriate”.

“This is a horrific shooting, we’re not to justice yet and there will be moments of energy and angst over the coming days and weeks,” he told CNN’s State of the Union.

Nixon had declared a state of emergency and a curfew starting at midnight on Saturday until 5am for the St Louis suburb, after looting and violence overnight Friday to Saturday.

The governor said he would decide later on Sunday whether the curfew would continue.

The latest violence began when riot police moved to break up a crowd of protesters who had gathered near where Brown was killed.

Missouri Highway Patrol Captain Ron Johnson, the African-American officer Nixon put in charge of restoring peace in Ferguson, said police acted after receiving reports of a shooting and that armed individuals had broken into a restaurant.

“We have a shooting victim in critical condition that may lose her life,” said Johnson, speaking to reporters.

“We had a subject standing in the middle of the road with a handgun. We had a police car shot at tonight. And, yes, I think that was a proper response tonight, to maintain officer safety and public safety.”

Seven people were arrested for failing to disperse, he said.

Antonio French, an area politician who was with the protesters when police moved in, wrote that some were ready for violence.

“I can tell you firsthand that some of the people that remained tonight were armed. Were ready for a fight. And wanted to injure police,” he wrote on Twitter.

He also wrote: “It’s important to differentiate the protesters from those violent opportunists that are not thinking about #MikeBrown or justice. #Ferguson.”

Brown’s family has accused authorities of a “devious” attempt to smear their son’s character after police named Brown as a suspect in the robbery of a Ferguson convenience store and released a surveillance video of the robbery.