Around 200 people were killed on Sunday when an arms dump exploded in Brazzaville, capital of the Congo Republic, a senior official in the presidency said, citing hospital sources.
Hundreds more were injured by the blasts which rocked the riverside capital of the oil-producing nation early in the day, flattening houses near the scene and sending a plume of smoke high above the city.
According to sources at the central hospital we're talking of around 200 dead and many injured, Betu Bangana, head of protocol in the president's office in Brazzaville, told Reuters by telephone.
Some people are still (trapped) in their houses... They're saying the entire neighbourhood of Mpila has been destroyed.
Defence Minister Charles Zacharie Bowao dismissed any talk of a coup attempt or mutiny, and told state radio that the explosions had been caused by a fire in the arms depot in the Regiment Blinde base in the riverside Mpila neighbourhood.
Panic also spread to Kinshasa, across the Congo River which separates the former French colony from the larger Democratic Republic of Congo, where windows were shattered by the force of the blasts around 700 metres (770 yards) away.
Both governments called for calm.
China's Xinhua news agency cited Chinese officials as saying three Chinese workers were killed by the explosion and dozens were injured, some in serious condition.
Xinhua said the dead and injured were part of a group of about 140 Chinese workers of the Beijing Construction Engineering Group.
A Reuters witness in Brazzaville said residents fled the blast area, which is near a heavily populated neighbourhood and was blocked off by the security forces as a military helicopter flew overhead.
Fleeing residents said houses in the area had been flattened.
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.
Congolese television showed pictures of panic-stricken people on the streets of nearby neighbourhoods. There were also images of many injured people being rushed to hospital or being given first aid on the street.
Television showed crowded hospitals where doctors said they were selecting those who were seriously injured to have immediate surgery. Television also relayed an appeal by health authorities urging all medical personnel living in Brazzaville to rush to hospitals in the city.
Mass in Brazzaville's cathedral, about 4 kms (2.5 miles) away, was cut short as the building shook.
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 who was in the city, told Reuters.
The main blasts were followed by a series of smaller ones. Hours after the main explosions, a plume of grey smoke still hung over the city, a Reuters witness in Kinshasa said.
Congo Republic has suffered coups and a civil war since independence from France. It has been mostly peaceful, however, since President Denis Sassou-Nguesso took power in a coup in 1997.
Bangana, from the president's office, said Sassou-Nguesso was not hurt by the explosions.
(Additional reporting by George Fominyen in Dakar; Writing by David Lewis; Editing by Ben Harding)