Huge explosions at an arms depot, which killed at least 146 people in Congo's capital, Brazzaville, were caused by a short-circuit that led to a fire, government officials say.
They say some 1,500 people were hurt.
A BBC reporter in the city says rescuers still searching for survivors are unable to get into the blast site because of unexploded shells.
The force of the blasts was felt several miles away in Kinshasa, in the Democratic Republic of Congo.
The BBC's Thomas Hubert in Brazzaville says there are fears that hundreds of bodies could be buried in the rubble of buildings surrounding the ammunition store.
Emergency workers say there are also fears that fires could spread to a second, bigger arms depot, close to the first one.
Residents in the east of Brazzaville fled when the first blasts occurred - and at least 2,000 people are now in temporary shelters throughout the city.
The explosions started shortly after 08:00 local time (07:00 GMT) on Sunday and continued into the early afternoon, causing panic in both cities - which are separated by the river Congo.
"It's like a tsunami passed through here. The roofs of houses were blown off," Congolese student Christine Ibata, was quoted as saying by the Associated Press news agency.
Government spokesman Bienvenu Okyemi said late on Sunday that a short-circuit led to a fire - which then quickly spread through the depot.
He added that the confirmed death toll stood at 146, although hospital sources have told the BBC it is more than 200.
The main fire was now under control, the spokesman said, though a number of buildings were still burning.
After visiting two hospitals, visibly shaken President Denis Sassou-Nguesso said the government was doing all it could and urged the Congolese "to show courage and solidarity".
He also announced a curfew in the area and set up an exclusion zone.
The government also asked "several international organisations" to provide assistance in dealing with the disaster, the AFP news agency reports.
Defence Minister Charles Zacharie Bowao earlier appeared on national TV to urge calm in Brazzaville and across the Congo river in Kinshasa.
"The explosions that you have heard don't mean there is a war or a coup d'etat," he said.
"Nor does it mean there was a mutiny. It is an incident caused by a fire at the munitions depot."
Streets in the capital were littered with twisted sheets of metal and debris.
Didier Boutsindi, of the presidential office, said many people were trapped in the wreckage of a collapsed church.
A worker for Congolese TV, speaking from hospital, told how he was injured amid the panic.
"I was at home and suddenly I heard this explosion coming from the camp," he said.
"There was panic. Houses and walls started collapsing and when we went outside, a wall fell on my head."
Across the River Congo in Kinshasa, windows were blown out and roofs damaged by the blasts.
Our correspondent who was in Kinshasa at the time of the blasts says many people fled from the river fearing shelling had broken out in Brazzaville.
China's Xinhua news agency said six Chinese workers were among those killed and several others had been injured, some seriously.
The Beijing Construction Engineering Group said about 140 Chinese workers were at a construction site near the scene of the blasts, the agency added.
Xinhua said the windows of its bureau office in Brazzaville had been blown out by the explosions.