Thousands of coastal homes have been levelled and trees uprooted as a powerful cyclone batters Bangladesh.
Many people are taking cover in state-provided cyclone shelters
Hundreds of thousands of people were evacuated or sought safe shelter before the storm hit the coast, but there are fears for those left behind.
The cyclone, which roared in from the Bay of Bengal just before dusk on Thursday, is packing winds of up to 240kph (150mph) and driving rain.
It is travelling north, and expected to hit Dhaka by noon on Friday (0830GMT).
It is also expected to affect eastern India and the west coast of Burma.
More than 40,000 policemen, soldiers, coastguards and health workers have been deployed along the coast.
The cyclone made landfall in an area of mangrove forests known as the Sundarbans - a world heritage site and home to rare royal Bengal tigers.
The hope was that the trees there would absorb most of the storm's strength but communities along the whole coast, including several large cities, are feeling its force, reports the BBC's Mark Dummett.
Most ordinary houses are made of thatch, wood and tin, and officials and local witnesses say many are being easily flattened by the wind.
"Many trees have been uprooted and houses and schools blown away," Mostofa Kamal, a district relief and rehabilitation officer, told the news agency AFP late on Thursday by phone from Barisal, north of the Sundarbans.
Later, Bishnu Prashad, a resident of the southern coastal district of Bagerhat, told the Associated Press news agency they were "sitting out the storm by candlelight".
Hundreds of thousands of coastal villagers have been evacuated, with thousands more moving into government-built cyclone shelters - raised concrete buildings - or "mud forts" - shelters inside mud walls built along the coast to resist tidal surges - AP reported.
Residents of the low-lying country had been warned of possible tidal surges at least 3m (10ft) high, and rivers flowing into the Bay of Bengal were said to be swollen and rising.
Authorities said they had sent food, medicine, tents and blankets to the affected areas.
"We have taken all precautions," Ali Imam Majumder, a senior government official, reportedly said.
In many places electricity and communication connections are down, and reports of casualties were confused.
Reuters news agency reported at least 28 fishermen - 16 Bangladeshi and 12 Burmese - were missing after their boats sank in the storm.
The news agency AFP said one elderly man had drowned when a river boat had capsized.
But there was no confirmation of the reports.
Work was stopped at the main ports of Mongla and Chittagong, which have been put on alert, along with the coastal town of Cox's Bazar. Flights have been cancelled, boats and planes removed from the storm's path, and schools and colleges closed.
In the Indian state of West Bengal, which borders Bangladesh, the minister for disaster management Mortaza Hossain told the BBC precautionary measures had been taken.
They included putting the military on alert, stockpiling food, and using loudspeaker announcements to tell residents to seek shelter.
Southern Bangladesh is often hit by cyclones, but experts say the latest one is a category four storm, the most powerful so far in the season.
Bangladesh developed a network of cyclone shelters and a storm early-warning system, after a cyclone killed more than 500,000 people in 1970.
Casualties from cyclones have been significantly reduced as a result, officials say.