Cyclone Sidr, which is moving north over the Bay of Bengal and packing ferocious winds of up to 200 kilometres (130 miles) per hour and torrential rains, is expected to hit land between 9:00 pm (1530 GMT) and midnight.

The first area to be lashed will be around the Sunderbans, a vast mangrove forest straddling the India-Bangladesh border. The coastal area is also home to many poor fishing communities and the endangered Royal Bengal tigers.

"We started evacuating people from vulnerable coastal areas and our volunteers are using microphones asking people to take shelter in cyclone shelters," said Asraf Shameen, an administrative chief in Bangladesh.

Officials also travelled by speedboat to islands to warn residents to evacuate. The area's many coastal islands, whose shifting sand banks are home to some of the world's poorest people, are thought most at risk from the storm.

"The houses are made of only tin, bamboo and straw, which cannot withstand storms," said another Bangladeshi district administrator, Mohammad Monjur-e-Elahi.

The cyclone, visible in satellite images as a huge swirling white mass over the Bay of Bengal, was set to unleash tidal surges of up to six metres (20 feet) in some areas, Bangladesh weather official Bazlur Rashid said.

"The storm is still strong and we expect it to hit in the evening. The most vulnerable districts are Khulna and Barisal," he said. The two areas are in southern Bangladesh, between the capital Dhaka and the Indian city of Kolkata.

In 1970, some half a million people died when a cyclone hit Bangladesh, while an estimated 138,000 people died in a cyclonic tidal wave in 1991.

The lower death toll in 1991 was attributed to a network of cyclone shelters and a warning system introduced after the 1970 disaster.

Fishing boats have been ordered to remain in port until further notice and in Khulna alone, 250,000 people were being moved to shelters and medical teams were in place.

Supplies of dried food were also being stockpiled and the army and navy were on stand-by, a Bangladeshi official said.

Chittagong airport and sea port, to the east of the cyclone's path, were also closed.

Officials were also on high alert in neighbouring India's coastal states of West Bengal and Orissa.

"The cyclone has a diameter of about 500 kilometres with a wall of clouds about 200 kilometres tall," Ladu Ram Meena, deputy director of the weather centre for India's eastern region, told AFP.

"Cyclone Sidr is moving very fast. The speed of the cyclone may intensify further towards evening."

Authorities in India have been told to halt rail and other transportation in some areas due to the likelihood of heavy floods, Meena added. Large-scale damage to power and communication lines was also expected.

Kanti Ganguly, a West Bengal state minister, told AFP that 50,000 people had been evacuated from the area, with a similar number still being moved to safer ground.

The cyclone is expected to fizzle out on Saturday over India's northeastern state of Assam and just south of the mountain kingdom of Bhutan.