"To protect the people and property of Ferguson today, I signed an order declaring a state of emergency and ordering implementation of a curfew in the impacted area of Ferguson," Governor Jay Nixon told reporters.

"This is not to silence the people of Ferguson or this region -- or others -- but to contain those who are drowning out the voice of the people with their actions."

During his press conference Nixon was repeatedly interrupted by locals angered by an apparent lack of accountability for the largely white police force responsible for Brown's death in the majority black area.

"Excuse me, governor, you need to charge that police officer with murder," said one, referring to the white officer who shot Brown. The comments were met with echoes of "Yeah!"

"Call for an investigation," said another, as palpable anger and frustration simmered in the church hall where the briefing took place. "Where's the indictment?"

Others demanded that police guard their homes and businesses.

"What we're doing now is not who we are," Missouri Highway Patrol Captain Ron Johnson, in charge of restoring peace to the restive town, said as he sought to calm the press conference.

"Yelling at each other is not going to solve that.

"I don't care who you are, how old you are, and I don't care if you were peaceful protesters last night or a rioter rioting, you'll still get the same answers. Tonight we will enforce it with curfew. We won't enforce it with trucks and tear gas. We'll communicate and tell you... it's time to go home."

Riot police fired tear gas and clashed with protesters overnight as renewed unrest came hours after police announced Friday that Brown had been the suspect in a robbery at a convenience store in Ferguson.

Nixon said that the FBI had boosted its presence in Ferguson in efforts to advance the investigation into Brown's death, bringing an extra 40 agents who were going door to door Saturday seeking additional witnesses.

After a calm Thursday night -- a marked break from violence that had characterized the four previous nights -- the announcements served only to ratchet up tensions again, with people targeting shops, reportedly including the one where Brown carried out the alleged robbery on August 9.

Three stores were looted in the fresh disturbances that also saw Molotov cocktails lobbed at police, CNN said, but there were also reports that locals, some armed, had stepped in to stop shops from being ransacked.

Police -- who have been accused of a heavy-handed response -- retaliated with tear gas, smoke bombs and rubber bullets, but they mostly stayed at a distance in armored vehicles and riot gear.

On Saturday afternoon, hundreds of people gathered peacefully near the scene of Brown's shooting, marking the exact moment he was shot a week ago.

Brown's death at the hands of an overwhelmingly white police force has renewed a national debate about relations between law enforcement and African Americans.

His family appealed for calm but after police said Friday he was the suspect in a robbery and released CCTV footage of it, the family accused authorities of a "devious" attempt to smear the character of their son, who had no criminal record and was about to start vocational college.

The robbery occurred just minutes before the policeman shot him dead, but muddying the waters, police said the officer stopped Brown for walking in the middle of the street, not for stealing.

Surveillance video showed a young black man carrying cigars out of a convenience store, and pushing another man who appeared to try and stop him.

In Harlem, New York, African American civil rights activist Al Sharpton criticized the video's release, accusing the police of sullying Brown's image in the public eye.

"Have we lost our decency when you don't even let people mourn their loved ones without you trying to smear them with things that have nothing to do with the situation?" he asked.

"Are you telling me that you have the right to run down somebody and kill him over three or four cigars?"

Police identified the officer who shot Brown as Darren Wilson, 28, a white, four-year veteran of the force with no disciplinary record.

Wilson's home in a mostly white town some 18 miles (30 kilometers) from Ferguson has been under police protection, but neighbors told The Washington Post that the officer got "spooked and took off pretty quickly before the name was announced."

There have been only tentative signs of the protests spreading to other areas of the United States. (AFP)