Fighting erupted in the southern part of Congo's capital Brazzaville on Monday, with heavy fire in opposition bastions between troops and unidentified assailants sending thousands of residents fleeing.

By late morning, AFP reporters saw streams of people panicked by the gunfire heading north away from districts loyal to the opposition, which is contesting President Denis Sassou Nguesso's recent re-election.

The reason for the clashes was not immediately clear and no official comment was immediately available.

According to several witnesses, the crackle of automatic gunfire began after 2:00am (0100 GMT) in Makelekele and Mayana districts, and continued without stop until dawn.

Several explosions were heard and two police stations reportedly torched in the restive run-down districts, strongholds of Congo's opposition.

"Soldiers came and told us to leave before it was too late, now we don't know where to go," said a 24-year-old student who gave her name as Mercie.

By 8:15am, the gunfire had become sporadic although witnesses across the Congo river in northern Kinshasa, capital of the neighbouring Democratic Republic of Congo, also reported hearing explosions. 

In mid-morning hundreds of police and troops, some in armoured vehicles, fanned out across the city's southern areas.

Security forces threw up roadblocks on the main road between the south and the city centre, stopping all cars for checks.

 'Terrified'

 "I couldn't stand the sound of the bullets and the heavy arms, I'm terrified," a 55-year-old called Jerome told AFP.

The clashes erupted as Congo's Constitutional Court examines results from the March 20 presidential election won by veteran leader Denis Sassou Nguesso but denounced by five defeated candidates who have alleged "massive fraud". 

Sassou Nguesso, a former paratrooper colonel in office for 32 years, was declared winner with over 60 percent of the vote.

Last week, several southern districts observed a strike called in protest over the results in a country of biting poverty where oil riches only benefit a fraction of the population.

Congo has been on edge since an October constitutional referendum that ended a two-term limit on presidential mandates, allowing the head of state to run again. 

Critics accuse the president of rampant corruption and nepotism, blasting the referendum result as a "constitutional coup".

Former colonial power France on Monday called for "restraint" and urged French citizens to stay at home.

Sassou Nguesso served as president from 1979 to 1992 and returned to power in 1997 following a civil war.

He won two successive terms in 2002 and 2009, but both elections were contested by opposition parties.