Heavy gunfire has been heard in the streets of Brazzaville, the capital of the Republic of Congo where troops have been deployed.
Witnesses told the BBC that military barracks were attacked on Monday morning.
An anonymous official told the AP news agency that police are fighting the so-called Ninja militia group.
It comes weeks after Denis Sassou Nguesso won a third term as president in a disputed poll.
Mr Sassou Nguesso has been in power since 1979, except for a five-year period after losing elections in 1992.
The opposition has said the March election was marred by “massive fraud”.
Reuters news agency reports young opposition supporters have been chanting “Sassou, leave!” on Monday morning and set fire to a local mayor’s office and police headquarters in the Makelekele neighbourhood.
Civil wars and militia conflicts have plagued the country.
The Ninjas were a main anti-government force in Congo-Brazzaville’s 1997-99 civil war.
They were loyal to former Prime Minister Bernard Kolelas and signed a peace deal with the government in 2003.