Relatives sobbed on shore as victims were placed in body bags and carried away, a Reuters witness said, and video footage shared widely online showed a packed multi-deck vessel listing sideways in calm waters before it overturned and pitched flailing passengers into the lake.
Reuters was not immediately able to verify the footage.
It was not clear how many people were still missing and regional officials gave contrasting death tolls.
The governor of South Kivu province said the death toll was 78 and that 278 had been on board.
"It'll take at least three days to get the exact numbers, because not all the bodies have been found yet," Governor Jean Jacques Purisi told Reuters.
The governor of neighbouring North Kivu province said 58 people had survived the accident and that 28 people were confirmed dead so far.
The boat capsized about 700 metres from port and the causes of the accident are being investigated, he said in a statement.
Deadly boat accidents are common in DR Congolese waters, where vessels are frequently loaded beyond capacity.
At a local hospital, one survivor said conditions were calm when the crowded boat overturned.
As others drowned around him, he struggled to stay afloat until he was rescued by DR Congolese troops.
"I saw people sinking, many went under. I saw women and children sinking in the water, and I myself was on the verge of drowning, but God helped me," said 51-year-old Alfani Buroko Byamungu from his hospital bed.
Crowds gathered at the port in Goma where the vessel was meant to dock.
Some wiped tears from their eyes or wept on the ground while others shouted angrily, blaming the authorities for allowing a long-running rebel conflict to cut off roads which they said had led to more overcrowding on boats.
"All of this is part of the consequences of the war ... They no longer make an effort to clear the enemies off the road so that it can become operational again," shouted Mushagulua Bienfait, a Goma resident who lost three relatives in the accident.
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