Large waves were already pounding beaches in the state of Andhra Pradesh more than a day before Cyclone Phailin was due to hit. Villagers were evacuated to schools in the north of the state and in neighbouring Odisha, while panic-buying drove up food prices.
Satellite images showed Phailin some 500 km (310 miles) off the coast in the Bay of Bengal and likely to make landfall this evening, with widespread flooding expected from surges.
The images showed the storm covering an area roughly half the size of India. Some forecasters likened its size and intensity to that of hurricane Katrina, which devastated the US Gulf coast and New Orleans in 2005.
There is disagreement about the power packed by Phailin, although most forecasters expected it to weaken upon reaching the coast. The Indian Meteorological Department described it as a "very severe cyclonic storm" with wind speeds of 210 to 220km per hour (130 to 135 mph) and resisted upgrading it to a stronger "super cyclone."
However, London-based storm tracking service Tropical Storm Risk said Phailin was a super cyclone and placed it in the most intense Category 5 of storms.