Parts of India's west coast received up to 594 mm (23 inches) of rainfall over 24 hours, forcing authorities to evacuate people from vulnerable areas as they released water from dams that were threatening to overflow.
"Unexpected very heavy rainfall triggered landslides in many places and flooded rivers," Chief Minister Uddhav Thackeray, who heads Maharashtra's state government, told journalists.
"Dams and rivers are overflowing. We are forced to release water from dams, and, accordingly, we are moving people residing near the river banks to safer places."
The navy and army were helping with rescue operations in coastal areas, he added.
A few dozen people were also feared to have been trapped in landslides in Satara and Raigad districts, said a state government official who asked not to be named.
Thousands of trucks were stuck on a national highway linking Mumbai with the southern technology hub of Bengaluru, with the road submerged in some places, another Maharashtra government official said.
Meanwhile, hundreds of villages and towns were without electricity and drinking water, he said.
Rivers were also overflowing in the neighbouring southern states of Karnataka and Telangana where authorities were monitoring the situation, government officials there said.
Seasonal monsoon rains from June to September cause deaths and mass displacement across South Asia every year, but they also deliver more than 70% of India's rainfall and are crucial for farmers.