Cyclone Mahasen has begun crossing Bangladesh’s southern coast, as people packed into evacuation shelters.
The storm hit Patuakhali district on Thursday with winds of up to 100 kph (60 mph), and was heading for the ports of Chittagong and Cox’s Bazar.
One person has been reported dead, Bangladeshi officials say.
Thousands of people have been ordered to evacuate low-lying areas in Bangladesh and Burma, and take shelter in cyclone centres.
However, some displaced people in Burma have resisted calls for them to evacuate camps in Rakhine state.
The United Nations has warned that 8.2m people could be at risk from Mahasen in Bangladesh, Burma and north-east India.