Rescuers worked throughout the night to look for victims in the rubble of two dilapidated apartment blocks which collapsed suddenly on Monday morning not far from the centre of the Mediterranean port city.
A third adjoining building partially collapsed on Monday night.
Prosecutors said the bodies of a man and woman were found separately Tuesday under the 15-metre (50-foot) pile of rubble on Rue d'Aubagne, a narrow shopping street which now resembles the scene of an earthquake.
Rescuers formed a human chain to remove the debris, stone by stone.
Interior Minister Christophe Castaner said five to eight people were missing — five residents and three others who may have been visiting in one of the buildings.
“We still have hope, even if that hope is fading. There could still be air pockets,” said a senior rescue official.
The two other apartment blocks, which were in such a bad state that they had been condemned, were boarded up and in theory unoccupied.
Google Maps images taken in recent months showed the collapsed buildings had large visible cracks in their facades.
People had been living in nine of the 10 apartments at number 65, while a shop occupied the ground floor.
Google Maps images taken in recent months showed the two collapsed buildings, in the working-class neighbourhood of Noailles, had had large visible cracks in their facades.
One of them had been condemned and, with its windows boarded up, was well-secured and in theory unoccupied, officials said.
'It could've been me'
A young bar waiter watched the scene with tears in his eyes, anxious for news of an Italian woman who lived in the building.
“She was a great girl, she used to come and study at the bar,” he said, without giving his name.
Abdou Ali, 34, came in search of his mother after she did not come to collect her youngest son from school on Monday afternoon.
“I haven't had any news,” he said, wandering among the rescuers.
Sophie Dorbeaux meanwhile told AFP she had left the block on Sunday night to stay with her parents because the building's structural problems meant her door, like several others, was not opening or closing properly.
“The walls had been moving for several weeks and cracks had appeared,” the 25-year-old philosophy student said.
“It could have been me,” she added, visibly shaken.
Marseille city authorities, who have evacuated and rehoused 100 residents from nearby buildings as a precaution, believe heavy rain may have contributed to the buildings' collapsing.
But the incident — rare in a major Western city — has already sparked a political row over the quality of housing available to Marseille's poorest residents.
The neighbourhood is home to many buildings in a similarly poor condition, some of them run by slum landlords.
“It's the homes of the poor that are falling down, and that's not a coincidence,” said local lawmaker Jean-Luc Melenchon, leader of the leftwing France Unbowed party.
Marseille authorities began a vast upgrade plan for the city centre in 2011.
But a 2015 government report said about 100,000 Marseille residents were living in housing that was dangerous to their health or security.