A French worker, 53, was killed and two of those hurt, a Briton and a US citizen, were seriously injured.
Fire-fighters said the accident occurred when the roof of the stage became unbalanced as it was being lifted by four cranes, toppling one of the cranes which crashed on to the structure below.
About 50 people from a range of nationalities were working to set up the structure, city sports official Richard Miron said.
Up to a dozen people are believed to have been underneath the stage when the accident occurred at around 1715 (1515 GMT).
The roof “started shaking and collapsing” gradually, said Marseille city councillor Maurice Di Nocera.
“Since it did not collapse right away, that allowed several people to get out, to avoid being hit,” he said.