US police fired smoke bombs and tear gas at a crowd which defied an overnight curfew in Ferguson, where a black teenager was shot dead by police last week.
About 150 protesters refused to disperse before a midnight (05:00 GMT) deadline in the St Louis suburb.
A police spokesman said tear gas was a "proper response" after a police car was shot at.
Captain Ron Johnson said a man was critical in hospital after a shooting.
Speaking just before 03:00 local time (08:00 GMT) he said that the police operation through the night was not in response to the curfew but to the violence that erupted near the Red The BBQ Man restaurant in Ferguson.
Seven arrests have been made. The circumstances of the shooting are not clear, police said.
"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," Captain Johnson said.
The move comes after a week of violent clashes between heavily armed local police and protesters.
Michael Brown, 18, was shot dead on a street in Ferguson on August 9.
Hundreds of protesters gathered on the main road in Ferguson in poor weather conditions hours before the curfew was due to go into force on Saturday evening.
Many left peacefully but others shouted that they would not abide by the curfew.
Police warned the remaining demonstrators that they would be arrested unless they left the area.
They then fired smoke bombs and tear gas, after which the protesters appeared to leave.
"The curfew is going to make things worse," protester Phonso Scott, 24, told Reuters news agency.
"I think the cops are going to get violent tonight, but they can't lock us all up."
Governor Jay Nixon said that although many protesters were making themselves heard peacefully, he would not allow a handful of looters to endanger the community.
"We must first have and maintain peace. This is a test. The eyes of the world are watching," Nixon said. "We cannot allow the ill will of the few to undermine the good will of the many."
He also said the US Department of Justice was bolstering its investigation of the shooting.
Missouri State Highway Patrol Captain Ron Johnson, who is in charge of security in Ferguson, said 40 FBI agents were going door-to-door to gather information about the incident.
Johnson gave a joint news conference with Governor Nixon on Saturday afternoon at a church in Ferguson, where they were repeatedly interrupted by angry locals.
The latest tensions flared on Friday night after Ferguson police chief Thomas Jackson revealed the name of the officer who shot Brown.
Police also released CCTV footage that it said showed Brown stealing a pack of cigars from a convenience store and intimidating its owner shortly before he was killed.
But Jackson said the 18-year-old was not stopped because of the incident and that the officer who shot him did not know he was a robbery suspect.
The BBC correspondent in Washington says Jackson's comments sparked bewilderment and anger in Ferguson.
Brown's family said they were "beyond outraged" by the video's release, criticising what they said was a police attempt to "justify the execution-style murder".
Civil rights leader Al Sharpton says a peaceful rally, led by Michael Brown's family, is planned for Sunday.