Troops have been deployed on the streets of Brazzaville, the capital of the Republic of Congo.
Witnesses say this comes after a military barracks was attacked.
An anonymous official told the AP news agency that police are fighting members of a militia called the Ninjas.
The Ninjas were a main anti-government force in the 1998-99 civil war.
It comes weeks after Denis Sassou Nguesso won a third term as president in a disputed poll.