Cyclone Roanu struck the Bangladesh coast on Saturday killing six people and forcing hundreds of thousands more to flee their homes as the storm unleashed strong winds and heavy downpours.
Authorities took more than 500,000 people into shelters as the cyclone made landfall just after noon local time, packing winds as strong as 88 kilometres per hour.
"It has struck the southern coastal areas of Barisal and Chittagong with a wind speed of 62-88 kilometres per hour," Omar Faruq, a government meterological department official told AFP.
"The landfall began just after noon. It will take another three-four hours to complete the landfall," he added.
Several villages were inundated on the Banshkhali coast in Chittagong after the cyclone triggered a 1.5 metre storm surge, the Red Cross's cyclone preparedness official Ruhul Amin told AFP.
"Thousands of villagers were forced to flee their homes after the storm surge flooded their villages," he said.
It came after the peripheral wind of the cyclone struck coastal areas early on Saturday morning, causing widespread devastation across the impoverished region.
Six people perished and hundreds of mud-and-tin houses were damaged in two southern districts, police said.
"A mother and her young child were killed after a landslide buried their hillside home at Sitakundu in Chittagong. The landslide was caused by heavy rains," Shah Alam, a police inspector told AFP, adding that another child died in the Chittagong city.
Two others were killed in Tajmuddin town on Bhola island in the coastal region while a woman in her 50s died under a flattened house in nearby Patuakhali, police said.
Thousands of homes were being evacuated as the cyclone bore down.
"So far we have moved more than 500,000 people to cyclone shelters," Reaz Ahmed, the head of Bangladesh's Disaster Management Department told AFP.
Disaster authorities have shut down sea and river ports and ordered fishing trawlers not to go out, while the meteorological department warned of landslides in southeastern hill districts.
Officials said on Friday night that they were prepared to move more than two million people to nearly 4,000 cyclone shelters in the country's south.