The procession of the famous Lalbagcha Raja Ganesh was underway, with viewers crowding the roadsides, when the mishap took place near Bharatmata Theatre, police said.
Excessive load must have caused the collapse, they said.
While one person died, the injured have been taken to government-run KEM hospital, said the civic authorities.
Many more are feared to have been injured. Immersion processions have started around the city since morning, today being the last day of Ganesh festival.