The fire broke out at around 1100 hrs and soon triggered an explosion, police sources said.
Forty people, a majority of them employed by contractors, were working in the "Train Second" unit at the LLDPE plant inside the Indian Petrochemical Corporation Ltd (IPCL) at the Nagothane facility when the incident happened, they said.
The plant, situated 40 km from here, was shutdown for maintenance work.
While 25 of the injured have been rushed to the Harkisondas Hospital in central Mumbai for treatment, the others are being treated in local hospitals, they said.