Crowds of people abandoned their homes and headed to areas away from the path of the water, especially to the beach resorts of Hua Hin, Phuket and Pattaya.
"All of these destinations are packed with Thais who have moved from Bangkok," said Tourism Authority of Thailand deputy chief Sansern Ngaorungsi.
He said domestic flights from Bangkok's Suvarnabhumi airport - the country's main air hub, which is still operating as normal - were also "very, very packed".
A steady flow of Thais and foreigners streamed into the capital's bus terminals as people sought to escape, while Bangkok's main train station was crowded and roads north and east out of the city were choked.
"I saw on the news that the water is getting closer. Maybe it's not going to come but I don't want to take a chance," 72-year-old Canadian Claude Kerrignan said as he waited to board a bus to Pattaya.
"Many people in my building have left already. Around where I live they have sandbags four feet (1.2 metres) high but if what they say is true it's going to be higher than that," he added.
Government offices, schools and some businesses were shut across Bangkok, where supermarkets have been running low on - and sometimes rationing - essential items such as bottled water and eggs as residents stock up.
The stock market and banks, however, were still open for business as normal.
Much of the country has already been ravaged by the three-month flood crisis caused by unusually heavy monsoon rains, which has left more than 370 people dead and millions of homes and livelihoods damaged.
A huge runoff from the north equivalent to 480,000 Olympic swimming pools is expected to reach the capital at the same time as seasonal high tides this weekend, the UN's Office for the Co-ordination of Humanitarian Affairs said.
Districts north of the centre have already been inundated, shutting down the city's second biggest airport, Don Mueang, which is currently serving as the headquarters of the country's Flood Relief Operations Centre (FROC).
Early on Thursday a power transformer exploded to cause a black-out, but Yingluck told reporters at the airport that the FROC would not be relocated.
She admitted the situation in the capital was critical, with the government preparing extra shelters for Bangkok evacuees.
The FROC on Wednesday evening urged Bangkok residents in flood-prone areas to leave the city, saying food and deliveries were expected to become more difficult.
The crisis is taking its toll on the lucrative Thai tourism industry, with some countries, such as Britain, advising against all but essential travel to Bangkok and other flood-hit regions.
Thousands of inundated factories have also been shut down, putting more than half a million people temporarily out of work and disrupting global supply chains.
Japan's Toyota Motor - which has already idled plants in the kingdom - on Thursday called off weekend production at four North American factories as the impact of the floods prompted supply shortage fears.
- AAP
