The country's worst flooding in half a century, caused in part by unusually heavy monsoon rain, has killed 373 people since mid-July and disrupted the lives of nearly 2.5 million, until now mostly in the north and central provinces.
But Prime Minister Yingluck Shinawatra, a novice politician who only took office in August, told reporters the crisis had now reached a critical point for Bangkok.
"It seems like we're fighting against the forces of nature, massive floodwater that is causing damage to several of our dikes," she said.
"The truth is, we need to let it flow naturally out to the sea, and what we can do now is to manage it, so that it flows slowly, otherwise everybody will suffer."
As Yingluck's voice started to tremble, reporters asked if she was crying.
"No, I haven't cried and I won't. I'll be strong to solve this problem for the Thai people. Right now we need to release floodwater to the sea as soon as possible and we need a quick rehabilitation plan," she said.
Traffic in central Bangkok was light as a five-day holiday, declared by Yingluck's government so people could leave Bangkok, began. However, a main road out of the city to the flood-free south was jammed.
Many people were going to the seaside towns of Hua Hin and Pattaya, where hotel rooms and homes to rent were hard to find.
"We're heading off to Hua Hin because people said it'll be difficult, we should leave. We plan to stay for at least three days and will monitor the situation. If it gets worse, we won't come back," resident Pornchai Tangsuwongthai told Reuters TV.
Bangkok, a city of at least 12 million people that accounts for 41 percent of GDP, is in danger from run-off water from the north coinciding with high tides on the Chao Phraya river, which is already at a record high level in places.
TV showed crowded domestic check-in counters at Bangkok's main Suvarnabhumi airport. Don Muang, the city's old airport now used mainly by budget carriers for internal flights, had to close on Tuesday and services switched to Suvarnabhumi.
At least seven huge industrial estates have also had to close to the north of Bangkok. The central bank has revised its growth forecast for southeast Asia's second-biggest economy to 3.1 percent this year from 4 percent as a result. The finance minister's projection is a gloomier 2 percent.
The defense ministry said 50,000 armed forces personnel were standing by with 1,000 boats and 1,000 trucks to help evacuate people. City authorities said a third district in the capital was in danger.
"Today Sai Mai area is getting worse. By lunchtime, Bangkok will announce the evacuation of people in the area, as we did with Don Muang and Bang Phlad yesterday," Bangkok governor Sukhumbhand Paribatra said in a televised statement.
Late on Wednesday, the governor warned that dikes might not hold and the city could be swamped. Yingluck has said floodwater could remain in the capital for up to a month.
Banks and financial markets will operate normally during the holiday from Thursday to Monday, although flooding has forced 295 branches to close, including 21 in Bangkok.
The stock market was up 2 percent at midsession. Recently it has reacted more to changing views on Europe's debt crisis than to the floods, although certain sectors have seen sharp moves. Banks have tended to fall while drinking water and building material firms have gone up.
Bangkok residents have stocked up on food, and items such as bottled water, instant noodles and even rice are now in short supply. Some shops have restricted customers to small quantities to prevent hoarding.
Street vendors who supply hot food to most locals were scarce in some districts.
Workers were busy bailing out water from the grounds of the Grand Palace, a Bangkok landmark.
The floods will take their toll on the tourism industry, which employs more than 2 million people and makes up 6 percent 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.
Britain warned against all but essential travel to Bangkok and 26 provinces currently affected by flooding. China urged its citizens not to go to Bangkok and told tourists "in the disaster zone" to leave immediately.
Tourists intrepid enough to brave the floods may also have to contend with crocodiles.
Thailand is reckoned to the world's biggest breeder of crocodiles and scores are reported to have escaped from farms during the floods. Several have been killed or captured in residential areas in the ancient capital of Ayutthaya.