More than two million people in India and Bangladesh have been evacuated as Cyclone Bulbul hits the Bay of Bengal.
The storm made landfall at midnight local time (18:30 GMT) on Saturday, near Sagar Island in Indian West Bengal, and is expected to unleash surges as high as 7ft (2m).
Services at many seaports and airports in the region were also suspended - including at the busy Kolkata airport.
Bangladesh's two biggest ports, Mongla and Chittagong, were closed and flights into Chittagong airport were stopped.
Shah Kamal, Bangladesh's disaster management secretary, told AFP agency that the evacuated residents had been moved to more than 5,500 cyclone shelters.
Mamata Banerjee, Chief Minister for the Indian state of West Bengal, tweeted before the cyclone made landfall urging people to stay calm.
"Please do not panic," she wrote. "Kindly remain calm and co-operate with the administration in its rescue and relief efforts. Be alert, take care and stay safe."