Cyclone Nivar hurtled into the Puducherry coast late on Wednesday night, uprooting trees and packing strong winds and rains as tens of thousands of people took refuge in shelters.
Cyclone Nivar made landfall at around 2.30 am with winds of up to 130 kilometres per hour.
Thousands of state and national emergency personnel were deployed in Tamil Nadu, Andhra Pradesh and Puducherry as authorities suspended power supply across several cities to prevent damage to the electricity grid.
Initially classified as a "very severe cyclonic storm" as it swirled in the Bay of Bengal, Nivar weakened after landfall into a "severe cyclonic storm", the India Meteorological Department (IMD) said.
It is forecast to move north and further weaken over the next few hours, the weather bureau added.
The danger "is not over yet as some part of the cyclone is still over the sea though center is over the land," the IMD tweeted.
The fierce winds uprooted trees and toppled electricity pylons while downpours lashed parts of the region, causing flash floods.
The coastal states have been bracing for the cyclone - the fifth-strongest category on India's scale of seven storm types - and more than a lakh people were evacuated from low-lying areas. Nivar has forced authorities to declare a public holiday till Thursday, close the Chennai airport and metro services.
Tamil Nadu Chief Minister E Palaniswami appealed to people to stay indoors as far as possible and said over 4,000 "vulnerable" locations had been identified and local officials have been told to ensure peoples' safety.