Residents fled Thailand's capital, Bangkok, on Thursday after authorities warned the city would soon be flooded and called a special five-day holiday to let people escape.
Traffic in central Bangkok was light as the holiday began but a main road out of the city to the flood-free south was jammed. Many people were heading for the seaside town of Pattaya, where hotel rooms and homes to rent were hard to find.
TV footage showed crowded domestic check-in counters at Bangkok's main Suvarnabhumi airport. Don Muang, the city's old airport that is now used for domestic flights, had to close because of flooding on Tuesday.
Banks and financial markets will remain open from Thursday to Monday. Shops in the capital tend to remain open on normal holidays.
Thailand's worst flooding in half a century, caused by unusually heavy monsoon rain, has killed at least 373 people since mid-July and disrupted the lives of nearly 2.5 million, with more than 113,000 in shelters and 720,000 people seeking medical attention.
The north and central industrialised provinces have been worst hit, with at least seven industrial estates forced to close to the north of Bangkok.
Now the capital itself is in danger from a combination of run-off water from the north and high tides on the Chao Phraya river, which is at a record level at certain points in the centre.
Late on Wednesday, the city's governor said "massive water" was on its way. The authorities have already warned that dikes may not hold and the city of at least 12 million people could be swamped.
"Now we're at a critical moment, we need to monitor the situation closely from 28-31 October, when many areas might be critical," Bangkok governor Sukhumbhand Paribatra told reporters.
"Massive water is coming."
Sukhumbhand said 90% of the northern Don Muang district was under water and another, Bang Phlad in the west, was in a critical situation. Fourteen city districts were threatened by floods and two more could be hit on Thursday, he said.
Bangkok residents have scrambled to stock up on food, and items such as bottled water, instant noodles and even rice are in short supply. Some shops have restricted customers to small quantities to prevent hoarding.
The economic damage is difficult to quantify, but the central bank has revised its growth forecast for southeast Asia's second-biggest economy to 3.1% this year and could cut it again on Friday.
The finance minister's projection is a gloomier 2%.
Prime Minister Yingluck Shinawatra, who said two weeks ago that Bangkok was likely to escape the floodwaters, said on Wednesday that it could be flooded for as long as a month.
"But we shouldn't face water as high as two or three metres or staying for two or three months as we've seen in other provinces," she told reporters.
The floods are expected to take their toll on Thailand's tourism industry, which employs more than 2 million people and makes up 6% of gross domestic product.
Tourism minister Chumphol Silpa-archa said arrivals could be 500,000 to 1 million below the government's target of 19 million this year.