The blaze erupted at around 10 AM on the second floor of Harish Chambers, a four-storey building near the Naval dockyard in Fort area of south Mumbai, a fire official said.
Eight fire engines have been pressed into service to douse the flames, he said. Hectic efforts were on to bring the fire under control, he added.
According to police, there is an office of a medical insurance firm on the floor where the fire occurred. The entire building has been evacuated, they said. Further details were awaited.