The furious cyclone 'SIDR' hit the south-western coastline of Bangladesh with full vigour with an intensity of 240 kmph, officials said.

"The wind suddenly blew up furiously as it appeared the core of the hurricane (cyclone) reached the confluence of the Baleswar River," the official BSS news agency quoted Navy Captain Abidur Rahman of Bangladesh Coastguard from the south-western Mongla Port.

Witnesses said the cyclone unleashed tidal surges of up to five feet in some areas as Met office sources said the storm appeared as a huge swirling white mass over the Bay of Bengal in their satellite images.

But the Met office warned the tidal surges could eventually lash the coastal areas with as high as 20 feet.

"The course it is likely to cut through the coast along the Baleswar river to Barisal region and from down there to Jessore and Faridpur on its way to the north-east through (central) Dhaka, parts of Comilla, Tangail and Mymensingh," a meteorologist said.

Despite the overnight evacuation campaign, thousands of people took makeshift refuge at the coastguard offices and police stations at the last minute as the storm approached with its all wrath.

Officials from Khulna said they snapped power lines in 13 southwestern districts as precautions after the peripheral winds started to brush the coast since evening.

Meteorologists said the 'SIDR' was feared to be the worst in decades in terms of intensity or wind speed while the greater Barisal and Khulna regions were likely to be its worst victims.

They said storm approached towards the coast with a wind speed of 220 kmph, which is packing up to 240 kmph, the highest since the 1991 cyclone when the highest wind speed was recorded at 225 kmph.

Global weather tracker Tropical Storm Risk classified it as "category 4" while the Met office earlier issued the highest "great danger signal" No.10 for the south-western Mongla and signal No. 9 for the south-eastern Chittagong and Cox's Bazar ports. 

Disaster management officials said tens of thousands of people were evacuated overnight in southern coastline as the cyclone gained extra intensity overnight. The Met office said they feared it to pound the low-lying areas with 15 to 20 feet surges above the normal astronomical tides.

Reports from the coastal districts said tens of thousands of people were moved towards safety overnight while the massive evacuation was still under way.

Chief Adviser Dr Fakhruddin Ahmed has ordered a maximum preparedness to minimise the cyclone damage as he chaired the National Disaster Management Council, comprising advisers, chiefs of three services, secretaries and other officials concerned.

An emergency inter-ministerial meeting chaired by Cabinet Secretary Ali Imam Majumdar earlier in the morning was informed that 6,20,000 people were moved out of their homes to safety from the vulnerable areas in 15 coastal districts while the officials were frantically trying to evacuate 32 lakh before the cyclone hit the coastline.

The government cancelled leaves of all concerned government employees in the coastal areas to join the cyclone preparedness and relief campaign while extra blankets, dry food and cash were sent to reinforce the local administrations to face the situation.

Residents of the coastline reached in Dhaka by telephone said they were experiencing high winds, which the Met office said was caused by peripheral gusts of the storm that reached the coast after Wednesday midnight.

The Red Crescent's Cyclone Preparedness Centre earlier said more than 40,000 of their volunteers carried out a massive evacuation using sirens, loudspeakers and whistles urging people to move out of their homes and move towards safety.

According to Red Crescent estimates some 1.20 crore people were vulnerable state across the coastline.

The Armed Forces Division has opened a "special operations" room to coordinate rescue and relief campaign while an Inter Service Public Relations spokesman earlier said troops were ready to respond to any call in line with standing orders described in the armed forces manuals.

Reports from the other coastal districts said several hundred fishing boats took shelter in the coastline while the authorities ordered temporary suspension of launch or ferry services on internal routes.

Flight operations from and to Chittagong were cancelled while Chittagong, Cox's Bazar and Mongla Ports' authorities asked the vessels at its berths to move away as soon as possible after completion of loading and unloading of cargoes and suspended further sailing of vessels from Thursday morning.

But local officials and fishermen's groups said despite the earlier warning signals, several hundred boats were yet to return to the coast since the "sea was not that much rough earlier".