Gunbattles rocked the capital of the Congo Republic on Monday, shattering a relative calm that had followed President Denis Sassou Nguesso's re-election in a disputed poll last month.
Fighting erupted in the southern part of Brazzaville, with heavy fire in opposition bastions between troops and unidentified assailants sending thousands of residents fleeing, AFP news agency reported.
AFP reporters saw streams of people panicked by the gunfire heading north away from districts loyal to the opposition, which is contesting President Nguesso's recent re-election.
According to several witnesses, the crackle of automatic gunfire began after 2am local time (01:00 GMT) in Makelekele and Mayana districts, and continued without stop until dawn.
Several explosions were heard and two police stations were reportedly torched in the restive run-down districts, strongholds of the opposition.
Following the violence, hundreds of police and troops, some in armoured vehicles, fanned out across the city's southern areas and threw up roadblocks on the main road between the south and the city centre.
The reason for the clashes was not immediately clear, but government spokesman Thierry Moungalla told Reuters news agency that former members of a militia that fought President Nguesso in a 1997 civil war were behind the violence.
Reuters reported that young opposition supporters, chanting "Sassou, leave!", also erected barricades near the main roundabout in Makelekele and set fire to the local mayor's office and police headquarters.
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The official, who spoke on condition of anonymity because he was not permitted to speak to the press, said militiamen known as "the Ninjas" were fighting with police.
The government battled the rebels in the early 2000s when they demanded a larger role in government and the military.
The Ninjas were also one of the main anti-government forces in the 1998-99 civil war. A peace deal was signed in 2003.
The violence comes on a day when the Constitutional Court published final election results showing that President Sassou Nguesso had won re-election.
Results given by the head of the court, Auguste Iloki, show that Sassou Nguesso won 60 percent of the March 20 vote, trailed by challenger Guy Brice Parfait Kolelas with 15 percent and Jean Marie Michel Mokoko with nearly 14 percent.
Sassou Nguesso has ruled the Central African oil producer for 32 of the past 37 years. He won re-election on March 20 after pushing through constitutional changes in an October referendum to remove age and term limits that would have prevented him from standing again.
Opposition candidates have described the election as a fraud and called for a campaign of civil disobedience.
A general strike last week was largely observed in southern Brazzaville but ignored in the capital's north, where Sassou Nguesso is popular.
At least 18 people were killed by security forces during opposition demonstrations before the October referendum.