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 was quoted by AFP as saying.

His statement was issued as an 18-year-old was charged in connection with a shootout in Ferguson on Sunday after a day of peaceful protests marking one year since the police shooting of unarmed black teenager Michael Brown.

Tyrone Harris is accused of first-degree assault on a police officer, armed criminal action and shooting at or from a motor vehicle, St. Louis County police said in a statement.

Harris, from Northwoods, another part of St. Louis County, remained in hospital Monday with wounds sustained in the shootout.