BBC
The city's bus and train stations and many roads are jammed by crowds of people attempting to flee.
People in several northern districts of the capital - some of which are now 90% submerged by rising water - have been told they should evacuate immediately.
More than 360 people have died in Thailand's worst flooding in decades.
The crisis is an early test for Prime Minister Yingluck Shinawatra, who took office in August and has previously been criticised for failing to take the flood threat seriously enough.
"It's a crisis, because if we try to resist this massive amount of floodwater, a force of nature, we won't win," Ms Yingluck said.
"But if we allow it to flow freely then people in many areas are prepared."
Any lingering sense of complacency has long gone, says the BBC's Rachel Harvey in Bangkok.
'Food rationing'
Over the weekend the influx of run-off water from Thailand's inundated central plains is expected to combine with seasonal high tides to flood more parts of the capital.
Thai authorities have declared a five-day holiday, to run from Thursday through to Monday, in Bangkok and in 20 provinces affected by the flooding to allow residents to relocate.
Thousands of Bangkok residents have been trying to leave the capital
Thousands of residents are rushing to leave the Thai capital Bangkok, which is braced for potentially severe flooding over the weekend.
People in several northern districts of the capital - some of which are now 90% submerged by rising water - have been told they should evacuate immediately.
More than 360 people have died in Thailand's worst flooding in decades.
The crisis is an early test for Prime Minister Yingluck Shinawatra, who took office in August and has previously been criticised for failing to take the flood threat seriously enough.
"It's a crisis, because if we try to resist this massive amount of floodwater, a force of nature, we won't win," Ms Yingluck said.
"But if we allow it to flow freely then people in many areas are prepared."
Any lingering sense of complacency has long gone, says the BBC's Rachel Harvey in Bangkok.
'Food rationing'
Over the weekend the influx of run-off water from Thailand's inundated central plains is expected to combine with seasonal high tides to flood more parts of the capital.
Thai authorities have declared a five-day holiday, to run from Thursday through to Monday, in Bangkok and in 20 provinces affected by the flooding to allow residents to relocate.
Continue reading the main story 
Those who can, appear to be heeding the advice to get out of Bangkok, at least for the next few days.
Many major roads leading north - towards the areas worst affected by the flooding - are largely impassable.
Those leading south and east are reported to be clogged with traffic, made worse by the number of cars that have been parked on elevated sections of roads to save them from the approaching waters.
Those who have decided to stay in the capital have been stocking up on essential provisions. Supplies of bottled water and dried noodles are running very low.
Satellite images now show Bangkok surrounded by a sea of water which is creeping deeper into the nation's capital every day.
Analysis
Many major roads leading north - towards the areas worst affected by the flooding - are largely impassable.
Those leading south and east are reported to be clogged with traffic, made worse by the number of cars that have been parked on elevated sections of roads to save them from the approaching waters.
Those who have decided to stay in the capital have been stocking up on essential provisions. Supplies of bottled water and dried noodles are running very low.
Satellite images now show Bangkok surrounded by a sea of water which is creeping deeper into the nation's capital every day.
Bangkok's Governor Sukhumbhand Paribatra issued an evacuation alert for residents of Don Muang, Bang Phlad and Thawi Wattana districts in the north of the capital.
The city's Mo Chit bus terminal has already seen huge crowds of people, some of whom had to wait for hours to get a bus to leave the city.
Many residents were reportedly heading for southern and eastern coastal resorts that have escaped the flooding.
Flights out of the international airport, Suvarnabhumi, were operating as normal but were said to be packed.
At Bangkok's domestic airport, Don Muang, people living in an evacuation shelter are now being moved again. The airport itself is closed because the runway is flooded.
But the government's flood relief operation centre, which is based on the second floor of the airport, insists it will not relocate.
Prime Minister Yingluck Shinawatra warned that parts of the capital could be flooded
Supplies of bottled water were said to be running low in many areas.
Residents were protecting their homes and businesses with sandbags - with some even erecting sealed cement barriers across shop fronts, reported the Associated Press news agency.
Britain's Foreign Office on Wednesday warned against all but essential travel to Bangkok.
Ms Yingluck has warned that flood waters could linger in the capital for between two weeks and a month.
The flooding has been triggered by unusually heavy seasonal rains across the country since July. About one third of all provinces remain affected by flooding.
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