FERGUSON, United States (AFP) - Two police officers shot overnight in the troubled Missouri city of Ferguson - which for months has battled racial tensions - were victims of an "ambush," police said Thursday.

One officer was shot in the face, and the other in the shoulder, as protesters outside the Ferguson police station - who gathered after the city's embattled police chief resigned - dispersed at around midnight.

The incident renewed tensions in the St Louis suburb, which has been on a razor's edge since the Aug 9 killing of unarmed black teenager Michael Brown by a white police officer.

"By God's grace, we didn't lose two officers last night," said St Louis County police chief Jon Belmar as detectives combed the scene of the shooting amid a search for suspects.

"This is really an ambush, is what it is," Belmar told reporters.

"That is something that is very difficult to guard against when you have a group of officers standing together in a large group and you have gunfire, certainly gunfire directed at them."

The shooting came just hours after Ferguson police chief Thomas Jackson resigned, in the wake of a blistering US Justice Department report that shed a damning light on racial bias in his department.

Yellow police tape surrounded the Ferguson police station Thursday morning as detectives combed the vicinity, looking for clues.