Hundreds of other protesters left peacefully before the midnight to 5 am deadline took effect in Ferguson, Missouri early today.
But remaining protesters - chanting "No justice! No curfew!" - refused to leave the area. As five armoured tactical vehicles approached the crowd, officers spoke through a loudspeaker: "You are in violation of a state-imposed curfew. You must disperse immediately. Failure to comply, may result in arrest."
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 smoke into the crowd of protesters.
Missouri governor Jay Nixon said yesterday that though many protesters were making themselves heard peacefully, the state would not allow looters to endanger the community of Ferguson, where 18-year-old Michael Brown was shot in a street.
"I am committed to making sure the forces of peace and justice prevail," Mr Nixon said during a press conference at a church that was interrupted repeatedly by people objecting to the curfew and demanding that the officer who shot Mr Brown be charged with murder.
"We must first have and maintain peace. This is a test. The eyes of the world are watching," Mr Nixon, who also declared a state of emergency, said.
State statute gives the governor broad powers when he or she declares a state of emergency, but Mr Nixon has not indicated that he plans to do anything other than impose the curfew and empower the state highway patrol to enforce it.
The curfew announcement came after tensions again flared in Ferguson. Earlier, local police identified the officer who shot Mr Brown as Darren Wilson and released documents and video footage claiming that Mr Brown had robbed a convenience store just before he was shot.
Police said Officer Wilson was unaware Mr Brown was a suspect when he encountered him walking with a friend.
Mr Nixon said the US Department of Justice was stepping up its civil rights investigation of the shooting.