Violence again erupted in the St Louis area near the site of the police shooting death of an unarmed black teenager, according to local police.
The fresh violence comes despite calls by US President Barack Obama and activists for a measured response.
Early this morning, a police officer shot and critically wounded a man who drew a handgun near the site of protests over the death of 18-year-old Michael Brown.
Police responded at about 1am to reports of four or five men with shotguns and wearing ski masks.
They encountered multiple suspects running, one of whom pulled a gun on an officer, who fired at him, a police spokesman said.
The man was taken to a local hospital.
Shortly after midnight, police fired tear gas at protesters who had confronted a line of officers after a far larger crowd dispersed, St Louis County Police Department spokesman Brian Schellman said.
A photograph in the St Louis Post-Dispatch showed a protester wearing a shirt with a US flag printed on it throwing a tear gas container back at the police. There were other media reports of bottles thrown at police.
The incidents followed two nights of violent protests, looting and arrests in Ferguson, the largely black St Louis suburb where the shooting of Mr Brown took place.
Mr Obama promised a full investigation by the US Department of Justice into the teenager's death.
"I know the events of the past few days have prompted strong passions, but ... I urge everyone in Ferguson, Missouri, and across the country, to remember this young man through reflection and understanding," Mr Obama said in a statement.
Friends and family of Mr Brown held a peaceful church vigil last night, after his father pleaded for an end to the violence.
Standing with supporters, including the Rev Al Sharpton, Michael Brown Sr said he wanted justice for his son but wanted it "the right way".
"I need all of us to come together and do this right, the right way," said Mr Brown Sr, who wore a T-shirt showing his son's baby picture. "No violence."
Several hundred protesters appeared to heed the calls for non-violence late yesterday, chanting "hands up, don't shoot" and "no justice, no peace" during a tense but ultimately peaceful stand-off with police clad in riot gear and flanked by armoured vehicles near the site of Mr Brown's death.
The protesters, some of whom waved signs as the group was led in chants by megaphone, had dwindled to a handful before midnight.
Rev Sharpton, a New York-based civil rights leader, called for peaceful protest in the wake of looting and more than 50 arrests since the shooting.
"To become violent in Michael Brown's name is to betray the gentle giant that he was," Rev Sharpton said of the 1.9m (6'4") student, who had planned to start college this week.
Police said Mr Brown was shot in a struggle with a gun in a police car but have not said why he was in the car.
At least one shot was fired during the struggle, and then the officer fired more shots before leaving the car, police said.
A witness to the shooting interviewed on local media has said that Brown had been putting his hands up to surrender when he was killed.
The FBI has opened a civil rights investigation into the racially charged case and St Louis County also is investigating.