Hundreds of others were injured by the blasts which rocked the riverside capital around 8 a.m. (0700 GMT), flattening houses near the scene and sending a plume of smoke high above the city.
A government spokesman said that a short circuit was to blame for the fire that sparked the explosions and promised to move military barracks out of town as a result.
But witnesses reported scenes of carnage on Sunday.
“I saw someone being carried to hospital with their intestines hanging out. They had been hit by a shell,” one witness told Reuters as he was leaving the blast zone.
A nearby church, packed with worshippers, collapsed, said one witness.
Corpses, many burned or with missing body parts, were carried into the main city hospital morgue, said a Reuters reporter outside the building. Officials there said they had already counted 136 bodies by mid afternoon.
Many more corpses littered the blast scene, said one soldier. Others are believed to be trapped under houses and other buildings that collapsed during the explosions.
Weeping relatives of the dead gathered outside the main hospital to mourn while others came to look for family members who had scattered in the chaos.
Congo’s government, which gave a toll of 146 dead, will pay for the treatment of all the injured and look after orphans, government spokesman Bienvenu Okiemy said.
Betu Bangana, an adviser in the president’s office, told Reuters earlier in the day that around 200 people had been killed before later revising the estimate down to 120.
“Some people are still (trapped) in their houses… They’re saying the entire neighbourhood of Mpila has been destroyed.”
Panic spread to Kinshasa, 4 km (2.5 miles) across the Congo River, which separates the former French colony of Congo Republic from the larger Democratic Republic of Congo. The blast was so strong it shattered windows in the neighbouring city.
Both governments called for calm.
Congo Republic Defence Minister Charles Zacharie Bowao quickly dismissed any talk of a coup attempt or mutiny, and told state radio the explosions were the result of a fire in the arms depot at the Regiment Blinde base near the city centre.
CHINESE KILLED
Residents immediately fled Mpila, normally a densely-packed neighbourhood, as a series of smaller explosions rang out, said a Reuters witness. A plume of grey smoke still hung over the city hours later as a military helicopter circled the blast zone.
The base and surrounding neighbourhood looked like a war zone. Many buildings were levelled, burned or badly damaged and the occasional flame still flickered in the debris. Some residents tentatively returned later in the day.
“I heard at least five or six good sized explosions, which blew out the windows and brought down half the ceiling in our hotel,” Patrick Mair, an analyst with Control Risks there, told Reuters.
China’s Xinhua news agency cited Chinese officials as saying three Chinese construction workers were killed and dozens injured, some in a serious condition. They were part of a group of about 140 Chinese workers from the Beijing Construction Engineering Group.
Congolese television earlier showed pictures of panic-stricken residents on nearby streets as the injured were rushed to hospital or received first aid on the spot.
At least 225 people were being treated at the main university hospital, a source there said.
Local television interviewed doctors saying they had to select who should receive surgery first. It also relayed an appeal for all medical personnel in the city to report for duty.
Mass in Brazzaville’s cathedral about 4 km (2.5 miles) away was abandoned as the building shook.
Oil-producing Congo Republic has suffered coups and a civil war since independence from France in 1960. However it has remained largely peaceful since President Denis Sassou-Nguesso took power in a coup in 1997.
Presidential adviser Bangana said Sassou-Nguesso had not been injured by the explosions.