The streets around the collapsed building were cordoned off and choked with dust. Firefighters could be seen attempting to work their way through the building debris.
"We are trying to speed up the (search and rescue operations) because time is of essence," but the search dogs cannot keep up in the heat, said the commander of the Marseille marine firefighters, Lionel Mathieu.
It remains unclear how many residents were in the collapsed building, authorities said.
But rescue workers are in a race against time to evacuate damaged neighbouring buildings also at risk of collapse.
"At the moment, we are clearing the buildings next door, taking care not to endanger the people who could be beneath the rubble," the mayor said.
In the surrounding buildings, more than 30 residents were "affected", while five were injured. All evacuees were currently sheltering in schools and nurseries.
Regional prefect Christophe Mirmand told AFP there were "strong suspicions" that an explosion caused the collapse, possibly a gas leak.
A late-night food vendor working on the street said "everything shook" during what sounded like an explosion.
"We saw people running and there was smoke everywhere," said Aziz, who declined to give his last name.
Gilles, who lives on a side street near the fallen building, told AFP the sound of the crash "was huge".
"It sounded like an explosion," said Gilles, also declining to provide his last name.
Another major structural collapse occurred in Marseille in November 2018, when two dilapidated buildings in the working-class district of Noailles caved in, killing eight people.
The accident cast a light on the city's housing standards.
But authorities appeared to rule out structural issues in the latest collapse, in a neighbourhood known for its bars and nightlife.
"There was no danger code for this building, and it is not in a neighbourhood identified as having substandard housing," said Mirmand.
