Tropical storm Pabuk has lashed southern Thailand with wind and rain, in what is expected to be the worst storm to hit the region in 30 years.
The storm made landfall at 12:45 local time (05:45 GMT), sending trees crashing down into houses in the province of Nakhon Si Thammarat.
It is moving across southern Thailand, affecting popular tourist spots.
Thousands of people have left Koh Samui, Koh Tao and Koh Phangan islands but many are riding out the storm.
At least two nearby airports on the mainland have been shut and ferry services have been suspended. Tourists stranded on Koh Samui have told the BBC that the island is seeing heavy rain, wind and waves and there have been short power cuts.
People also say they have been advised to stay indoors after 16:00 local time until Saturday morning.
"I talked to foreigners last night and they are not scared, they understand the situation," Krikkrai Songthanee, the district chief of neighbouring Koh Phangan, known for its raucous parties, told the AFP news agency.
He said 10,000 tourists were still on the island.
The eye of the tropical storm is passing over Nakhon Si Thammarat, to the south, and is expected to weaken into a tropical depression over Surat Thani province.
"But all tourist islands in the Gulf of Thailand including Koh Samui, Koh Phangan, Koh Tao will be affected because Pabuk is huge," meteorological department chief Phuwieng Prakhammintara said.
One fisherman is reported to have died early on Friday in Pattani province, near the Malaysian border, as waves engulfed his boat.
The UK Foreign Office is advising Britons to avoid all but essential travel to provinces near the Thai-Malaysia border.
Daniel Moroz, from Bristol, said the situation on Koh Phangan was "fairly calm".
"It's just persistent rain at the moment but I've been told the worst is coming later. We've been reassured our place is safe and I think by tomorrow afternoon it should have cleared," he said.
"What's weird is that the locals aren't too bothered at all as I think the other islands will be hit more than here."
While storms are common in the Gulf of Thailand at this time of year, Pabuk is thought to be the worst in decades to hit during the peak holiday season.
In the past, tropical storms have killed hundreds of people but authorities say they are well prepared and able to cope with the expected storm surges of up to five metres.
The last big storm, Typhoon Gay, struck in 1989 and killed hundreds of people.
Thailand's meteorological department said the storm would affect the south of the country for the next few days and that "widespread rainfall, and torrential downpours are possible [in] much of the area".
"People should beware of the severe conditions that cause forest runoffs and flash floods," the authority warned.
The storm will weaken as it crosses over from the Gulf of Thailand to the Andaman Sea, but tourists there are also being warned to be prepared for heavy rain, rough seas and landslides.
In Thasala district of Nakhon Si Thammarat, a thousand evacuees had to be relocated to a university gymnasium after the government shelter they were in was damaged by the storm.
In the nearby district of Kanom, a businessman told BBC Thai his 1,000-room hotel had to be closed temporarily. He was optimistic that tourists would return after the storm was over.