Cyclone Mocha began to crash ashore at the Bangladesh-Myanmar border on Sunday, Bangladesh's weather office said, uprooting trees and bringing driving rain to a region home to hundreds of thousands of Rohingya refugees.
Packing winds of up to 195 kilometres (120 miles) per hour Mocha hit between Cox's Bazar, where nearly one million Rohingya refugees live in camps largely made up of flimsy shelters, and Myanmar's Sittwe, the office said.
The US Joint Typhoon Warning Center earlier said Mocha was packing winds up to 140 knots, or 259 kph, equivalent to a category 5 hurricane on the Saffir-Simpson Hurricane Wind Scale.
"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 Bangladesh.
"The schools, which are designated as cyclone shelters... are not strong shelters that can withstand the winds of a cyclone. We are scared."
"The wind is getting stronger at the moment," rescue worker Kyaw Kyaw Khaing told AFP earlier from Myanmar's Pauktaw, about 25 kilometres inland from Sittwe, and where he said around 3,000 people had arrived to seek shelter.
"We distributed enough food for one or two meals to the people evacuated to temporary shelters. I don't think we will be able to send any food today due to the weather."
Thousands left Sittwe on Saturday, packing into trucks, cars and tuk-tuks and heading for higher ground inland as meteorologists warned of a storm surge of up to 3.5 metres (11 feet).
A media account run by junta authorities in Rakhine showed what it said were trees downed over a road near Sittwe.
"We are not OK because we didn't bring food and other things to cook," said Maung Win, 57, who spent the night in a shelter in Kyauktaw town. "We can only wait to get food from people's donations."
Bangladeshi authorities moved 190,000 people in Cox's Bazar and nearly 100,000 in Chittagong to safety, divisional commissioner Aminur Rahman told AFP late Saturday.
The rain and wind were felt in Myanmar's commercial hub Yangon, around 500 kilometres away, residents said Sunday.
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 incentivise 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 a displacement camp in Myanmar's Kyaukphyu told AFP.
"A house at the camp collapsed and the roof of a shelter built by UNHCR was blown away," they said, requesting annonymity.
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 are in a panic because we don't have enough cyclone shelters here," Jahangir Sarwar, 23, a resident of the Saint Martin's told AFP by phone.
"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.
In 2008 Cyclone Nargis devastated Myanmar's Irrawaddy Delta, killing at least 138,000 people.
Scientists have warned that storms are becoming more powerful as the world gets warmer because of climate change.