Cyclone Mocha crashed ashore in Myanmar and southeastern Bangladesh on Sunday, uprooting trees, scattering flimsy homes in Rohingya displacement camps and bringing a storm surge into low-lying areas.
Packing winds of up to 195 kilometres (120 miles) per hour, Mocha hit between Cox's Bazar, home to nearly one million Rohingya refugees in Bangladesh, and Myanmar's Sittwe, according to Bangladesh's weather office.
Streets in Sittwe were turned into rivers as the biggest storm to hit the Bay of Bengal in more than a decade surged through the seaside town.
"The water is gradually rising," social worker Wai Hun Aung told AFP from Sittwe.
"The tide has reached to the drain in front of a school... Soon we will move our important belongings upstairs."
The wind ripped apart homes made of tarpaulin and bamboo at one camp for displaced Rohingya at Kyaukphyu in Myanmar's Rakhine state.
Its residents were anxiously watching the rising sea tide, camp leader Khin Shwe told AFP.
"We are now going to check whether sea water is increasing to our place... if the sea water rises, our camp can be flooded," Khin Shwe said.
In Teknaf in Bangladesh, high winds uprooted trees, brought traffic to a halt and sent residents running for cover, an AFP correspondent said.
"Our camp houses, which are constructed with bamboo and tarpaulins, can be blown away in soft, light winds," Mohammad Sayed, 28, told AFP from Nayapara refugee camp in Cox's Bazar.
The Myanmar Red Cross Society said it was "preparing for a major emergency response".
In Bangladesh, authorities have banned Rohingya refugees from constructing concrete homes, fearing it may encourage them to settle permanently rather than return to Myanmar, which they fled five years ago following a brutal military crackdown.
The camps are generally slightly inland but most of them are built on hillsides, exposing them to the threat of landslides.
Forecasters expect the cyclone to bring a deluge of rain, which can trigger landslips.
"The wind started about 8:30 this morning and it's getting stronger," a Rohingya community leader in the Kyaukphyu displacement camp told AFP.
"A house at the camp collapsed and the roof of a shelter built by UNHCR was blown away," they said, requesting anonymity.
Hundreds of people also fled Bangladesh's Saint Martin's island, a local resort area right in the storm's path, with thousands more moving to cyclone shelters on the coral outcrop.
Those left behind said they feared the storm's approach.
"We asked the administrators many times that everyone should be evacuated to a safe place in mainland Teknaf town. But no action was taken."
Cyclone Mocha is the most powerful storm to hit Bangladesh since Cyclone Sidr, Azizur Rahman, the head of Bangladesh's Meteorological Department, told AFP.
Sidr hit Bangladesh's southern coast in November 2007, killing more than 3,000 people and causing billions of dollars in damage.
Operations were suspended at Bangladesh's largest seaport, Chittagong, with boat transport and fishing also halted.
Cyclones -- the equivalent of hurricanes in the North Atlantic or typhoons in the Northwest Pacific -- are a regular and deadly menace on the coast of the northern Indian Ocean where tens of millions of people live.
Cyclone Nargis devastated Myanmar's Irrawaddy Delta in 2008, killing at least 138,000 people.
Scientists have warned that storms are becoming more powerful as the world gets warmer because of climate change.