Police have fired tear gas and issued warnings for crowds to disperse as a curfew starts for a second night in Ferguson in the US state of Missouri,It is in force from midnight (05:00 GMT) to 05:00 (10:00 GMT).
Police wearing body armour and gas masks said “aggressors” in a crowd of several hundred protesters were advancing on a police command post.
The suburb of St Louis has been tense for more than week, since police shot dead a black teenager, Michael Brown.
US Attorney General Eric Holder has ordered a federal post-mortem on the body of Michael Brown, 18, to take place “as soon as possible”.
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.
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.
The Quick Trip is the convenience store on a petrol station forecourt in Ferguson, wrecked in violence a week ago.
They call it the Ground Zero of the unrest in Ferguson.
Parked there after 21:00 on Sunday evening, we could see clouds rise up behind it as police began to disperse the protests on West Florissant Avenue, with a helicopter beaming down a spotlight.
We could also hear the sirens of the acoustic weapons used to clear people out.
When “flash-bangs” exploded on the forecourt, a bit like a ground-level firework display, designed to move people on, we drove down a side street as the crowd ran towards us.
Several of them were spluttering and pouring bottles of water into their eyes as they felt the effects of tear gas.
We saw one young woman collapse to the floor as she struggled to breathe, although she recovered some time later.
Protesters around me at an intersection on the edge of the neighbourhood are asking why the police dispersed what they say was a peaceful protest, more than two hours before the scheduled curfew.
He said the release of the footage “appeared to cast aspersions” on the dead man, who was not stopped by police on suspicion of robbery when he was shot.
“It made emotions raw,” Mr Nixon told US broadcaster ABC on Sunday.
The killing of Mr Brown by a white policeman in a street on 9 August has inflamed racial tensions in the largely black suburb.
Seven arrests were made on Saturday night as police used smoke bombs and tear gas to disperse a crowd of about 150 in Ferguson, after being called to the scene of a disturbance at a restaurant after the curfew started.
A police car reportedly came under fire and a person was hurt in an apparently unrelated shooting in the same area.
As police were nearing the restaurant, a man with a handgun went into the street but ran away, AP news agency reports.
Someone also shot at a police car but it was not clear if it had been hit.
Demonstrators complained that the curfew would make matters worse.
The US justice department is investigating the shooting of Mr Brown, with 40 FBI agents dispatched to Ferguson to gather information.
The justice department said on Sunday that Mr Holder had ordered a federal post-mortem examination at the request of Mr Brown’s family, in addition to a Missouri state autopsy.
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 in a private autopsy report, Dr Baden said Mr Brown was shot four times in his right arm and twice in his head, the New York Times revealed.
Ferguson police chief Thomas Jackson revealed on Friday that the officer who shot Mr Brown was Darren Wilson, a six-year police veteran with no previous complaints against him.
Mr Wilson has been on paid administrative leave since the shooting.
The policeman had reportedly stopped the teenager for walking in the street, disrupting traffic.