Saturday marked a week since a white Ferguson officer, Darren Wilson, shot and killed 18-year-old Michael Brown on Aug. 9. The shooting ignited racial tension in the mostly-black suburb and has led to looting and several run-ins between police and protesters.
Patrol Capt. Ron Johnson said the timing of police action on Sunday was coincidental.
Police received a report that people broke into a barbecue restaurant and some were on the roof - creating a potential danger for officers trying to disperse the protesters. Police were responding to that report - not the fact that protesters were still on the street after curfew, Johnson said.
Things got worse when a man with a handgun went into the street as police were nearing the restaurant. He ran away, but there was plenty of violence. A man was shot and critically wounded in the same area. Police were searching for the shooter. Someone shot at a police car - it wasn't clear if it was hit.
Hundreds of other protesters left peacefully before the curfew took effect. But remaining protesters - chanting "No justice! No curfew!" - refused to leave the area.
As officers put on gas masks, a chant from the distant crowd emerged: "We have the right to assemble peacefully." A moment later, police began firing canisters into the crowd of protesters.
Highway Patrol Spokesman Lt. John Hotz initially said police only used smoke, but later told The Associated Press that they also fired tear gas canisters.
Jayson Ross, who was leading the protesters toward police before the canisters were fired, said: "They got guns. We got guns. We are ready."
Gov. Jay Nixon on Saturday declared a state of emergency in Ferguson. The curfew announcement came after tensions again flared in Ferguson late Friday night. Earlier that day, local police identified the officer who shot Brown as Darren Wilson and released documents and video footage alleging that Brown had robbed a convenience store just before he was shot. Police said Wilson was unaware Brown was a suspect when he encountered him walking in the street with a friend.
Nixon said the U.S. Department of Justice is widening its civil rights investigation of the shooting.
Missouri State Highway Patrol Capt. Ron Johnson, who is in charge of security in Ferguson, said 40 FBI agents were going door-to-door in the neighborhood starting Saturday, talking to people who might have seen or have information about the shooting.
In announcing the curfew, Nixon said that though many protesters were making themselves heard peacefully, the state would not allow looters to endanger the community.
Darrell Alexander, 57, worried Saturday night that the curfew might spur anger and more violence.
"I think it's an antagonistic decision to not allow people to express their freedom of speech. It's an overreaction," he said.
Members of various black community groups were urging people to abide by the curfew, which runs from midnight to 5 a.m. Sunday, reports CBS affiliate KMOV-TV in St. Louis. A woman from the New Black Panther Party walked the street with a bullhorn, telling the crowd: "Please, please be out of the area by 12 o'clock."
Some responded to her pleas by cursing at police, while others acknowledged they planned to leave before midnight.
But Keyon Watkins, a 26-year-old computer science worker from St. Louis, said that if many others stay in the street, he would join them.
"All of this is just building up -- pent-up aggression by being mistreated on a daily basis," Watkins said.
On Saturday, some residents said it appeared the violent acts were being committed by people who came from other suburbs or states.
Wilson, the officer who shot Brown, is a six-year police veteran who had no previous complaints against him, the local police chief has said.
The Ferguson Police Department has refused to say anything about Wilson's whereabouts, and Associated Press reporters were unable to contact him at any addresses or phone numbers listed under that name in the St. Louis area.
Wilson has been on paid administrative leave since the shooting. St. Louis County prosecutor Bob McCulloch said it could be weeks before the investigation wraps up.