St Louis County has declared a state of emergency following a night of unrest in Ferguson, after a teenager was charged with shooting at police officers.
In a statement, St Louis County executive Steve Stenger said county police would immediately take charge of "police emergency management" in Ferguson and surrounding districts.
"In light of last night's violence and unrest in the city of Ferguson, and the potential for harm to persons and property, I am exercising my authority as county executive to issue a state of emergency, effective immediately," he said.
His statement was issued as an 18-year-old was charged in connection with a shootout in Ferguson on Sunday (local time) after a day of peaceful protests marking the first anniversary of the police shooting of unarmed black teenager Michael Brown.
Tyrone Harris is accused of first-degree assault on police officers, armed criminal action and shooting at a motor vehicle, St Louis County police said.
Harris, from Northwoods, another section of St Louis County, remained in hospital Monday with wounds sustained in the shootout.
Meanwhile, in St Louis city centre, more than 50 protesters were arrested after climbing the barricade around a federal courthouse during a midday demonstration, local news media reported.
Sunday's gunfire followed the looting of at least two Ferguson businesses, on a commercial strip around the corner from where Brown fell.
"I think it's unfortunate that a beautiful day of events ended like that," said Dellena Jones, whose hair salon was among the shops hit.
"Some people broke in to some of the businesses on our lot, and so we are all here helping each other," she told AFP on Monday.
Attorney-General Loretta Lynch strongly condemned Sunday's violence, as she spoke Monday at a police union convention in Pittsburgh, Pennsylvania.
"Not only does violence obscure any message of peaceful protest, it places the community, as well as the officers who seek to protect it, in harm's way," she told the Fraternal Order of Police gathering.
St Louis County Police Chief Jon Belmar earlier said the shootout started when four plain clothes detectives in a van came under fire.
AFP video showed a black man lying face down on the ground in handcuffs, bleeding profusely. Belmar declined to comment on the race of the detectives.
Sunday's day of remembrance had been peaceful until a handful of protesters grew rowdy later in the evening.
A crowd of about 300 people had gathered earlier to mark the anniversary, during which they observed four and a half minutes of silence and released two white doves.
The time was meant to symbolise the four and a half hours that Brown's body lay face down in the residential street where he was shot before being taken away.