Heavy gunfire erupted between the army and unidentified gunmen Monday in the Congolese capital Brazzaville, officials and witnesses said.
Communications Minister Thierry Mougalla said three police stations and an administrative building had been set on fire, but added that the situation was now under control.
The authorities were investigating what they called "terrorist action" and trying to identify the attackers, he said on national television.
Witnesses said government opponents chanted slogans hostile to President Denis Sassou Nguesso, who won a controversial election on March 20.
Hundreds of people reportedly fled the southern part of the capital, where the violence took place.
Soldiers were seen patrolling the streets while army helicopters circulated above.
Witness Nelson Cishugi said the attackers were believed to belong to the so-called Ninjas, an opposition militia which participated in the 1997-1999 civil war.
Sassou Nguesso first became president 37 years ago and has been in power in the West African country for a total of 32 years.
He pushed through constitutional changes in an October referendum which allowed him to extend his rule.
The opposition has called the election fraudulent and called for peaceful civil disobedience.