* Thousands flee fighting in south of Congo capital

* Calm restored but residents fear new flare-ups

* Government blames violence on ex-rebel militiamen (Updates events on ground, adds validation of presidential results)

By Christian Elion

BRAZZAVILLE, April 4 (Reuters) - Gunbattles shook the capital of Congo Republic on Monday, shattering a relative calm that had followed President Denis Sassou Nguesso's re-election in a disputed vote last month.

Former members of the "Ninja" militia that fought Sassou Nguesso in a 1997 civil war raided and set alight military, police and local government offices but the attacks have been contained, government spokesman Thierry Moungalla said.

Gunfire had died down by mid-afternoon as security forces blocked access to southern Brazzaville, where exchanges of heavy calibre fire between police and unidentified fighters broke out around 3 a.m. local time (0200 GMT), witnesses said.

The government did not say whether anyone was killed in the fighting. Thousands of residents streamed north, many carrying their possessions on their heads.

Sassou Nguesso, who has ruled the oil-producing central African country for 32 of the past 37 years, regularly presents himself as a bulwark of stability in a turbulent region. State-run television on Monday broadcast images of damage from the fighting alongside reminders of Congo's violent past.

The fighting between security forces and unidentified gunmen was some of the worst to hit Brazzaville since 1997, when Sassou Nguesso returned to power after months of urban warfare between rival militia groups in the capital. He had previously ruled the country from 1979 until 1992, when he lost an election.

Sassou Nguesso 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.

Some residents of southern Brazzaville who had taken shelter in churches in the centre and north of the city on Monday said they feared a resurgence of fighting after the government cast suspicion on the losing candidates in the election.

"The government ... does not yet have proof that the candidates or their supporters were involved in this affair but ... investigations are under way," Moungalla said on state television early in the afternoon.

Opposition leader Guy-Brice Parfait Kolelas, whose father led the Ninjas during the civil war, came second in the election. The Ninjas signed a peace accord with the government in 2003 after years of sporadic clashes, though rivalries persist along regional and ethnic lines.

Kolelas was not involved in the latest violence, an aide said. Residents of southern neighbourhoods said that they had seen armed men in civilian attire but could not say whether they were indeed former Ninja militiamen.

Opposition supporters are frustrated in the wake of the election by Sassou Nguesso's unwavering stranglehold on power. Young protesters chanted "Sassou, leave!" and erected barricades in southern Brazzaville's Makelekele neighbourhood as fighting raged in the morning.

Dozens of heavily-armed Republican Guard troops and police later headed towards the nearby Kingouari neighbourhood, where isolated gunfire persisted into the afternoon.

The Constitutional Court confirmed Sassou Nguesso's victory on Monday evening in a statement read on state television.

At least 18 people were killed by security forces during opposition demonstrations against the referendum changes.

Opposition candidates say the March vote was fraudulent and have called for a campaign of civil disobedience. A general strike last week held in southern Brazzaville was ignored in the north of the city, where Sassou Nguesso is popular.

The U.S. State Department said after the election it had received numerous reports of irregularities and criticised the government's decision to cut all telecommunications including Internet services during voting and for days afterwards.

Congo's election has been closely watched across Africa, where several long-ruling presidents are seeking to stay on beyond constitutionally mandated term limits. (Additional reporting and writing by Aaron Ross in Kinshasa; Editing by Matthew Mpoke Bigg and Mark Heinrich)