Thousands are fleeing Bangkok after the city's governor ordered residents of three districts to evacuate and breaches in dykes allowed floods to spread through the capital.
The central government has declared a five-day holiday from Thursday, encouraging many to leave the city. A naval official warned that Bangkok's main river is likely to overflow its defences on Saturday, when high tides are expected to merge with the heavy runoff from further north.
The floods are the country's worst for more than half a century and have claimed 373 lives. Unusually heavy rains have killed hundreds more in neighbouring Cambodia and Burma.
"We're at a critical moment, we need to monitor the situation closely from 28 to 31 October, when many areas might be critical … massive water is coming," said Bangkok's governor, Sukhumbhand Paribatra.
The UK Foreign Office urged Britons to avoid all but essential travel to the Thai capital and other flood-affected areas. China urged its nationals to leave the area.
The director general of the Thai navy's hydrographic department said the Chao Phraya river's water level – already at a record height – is expected to peak at 2.65 metres (9ft) after 6pm on Saturday, the Bangkok Post reported. Defences are 2.5 metres high.
Vice Admiral Nirut Hongprasit said the peak should last no more than 30 minutes, the newspaper said.
Thousands of people packed Bangkok's Mo Chit bus terminal and large crowds arrived at Suvarnabhumi airport. Bangkok's second airport, Don Muang, has closed to commercial flights owing to flooding.
Some roads out of the city had reached gridlock, with many better-off residents heading for seaside resorts such as Pattaya and Hua Hin.
In the Sai Mai district of Bangkok, residents clamoured to board evacuation trucks.
"We haven't been able to get on one [military truck] yet, we have been waiting for almost an hour," said 71-year-old Saman Somsuk. "There aren't many trucks."
Others escaped using paddle boats, plastic tubs, inner tubes and rubber rafts. On one flooded road, several men were floating on a plank buoyed up by empty barrels.
Around 50,000 soldiers were on standby to help evacuate residents, the defence spokesman told the Bangkok Post, with around 1,000 boats and 1,000 trucks.
Colonel Thantathip Sawangsaeng said more than 100 schools would be opened as evacuation centres taking up to 10,000 people from affected districts.
Yingluck Shinawatra, who was elected prime minister two months ago, told reporters the crisis had reached a critical point.
"It seems like we're fighting against the forces of nature, massive floodwater that is causing damage to several of our dykes," 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."
Reporters asked if she was crying as her voice began to tremble during the briefing.
"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.
Workers rushed to tackle breaches in flood barriers today, while residents in other districts built cement walls and stacked sandbags to protect homes and businesses. One woman carried a bag of life jackets as she travelled on the Skytrain.
Other parts of the region have suffered owing to heavy rains.
In Burma, at least 147 people died in a flash flood in Pakokku in the central Magway region last week. In one township, 2,200 homes were reportedly swept away.
In Cambodia, the most severe floods for a decade have left 250,000 people homeless, with water levels receding only very slightly since heavy rains ended. The National Committee for Damage Management said earlier this month that the disaster had caused $100m (£62m) in damages.
Kim Rattana, executive director of the charity Caritas, told the Phnom Penh Post that flooding had killed 247 people and affected more than one million others across 17 of the country's 24 provinces.
"We are seeing disease outbreaks in increasing numbers," warned John Macgregor of the Cambodian War Amputees Rehabilitation Society, which has been aiding victims in Battampang province in the north-west.
"I would say 80% of the children we have been dealing with have [diarrhoeal diseases]. There are skin rashes, pneumonia and colds … The conditions are wretched: they are sleeping on rush mats on mud under bits of plastic sheeting, with animals. You have got oxen, chickens, dogs and cats and people all crowded into tiny strips of land because the entire district is under water.
"Food and fresh water has been the primary need for some weeks, but that has now been equalled by the need for doctors and medicine – people survived for a while and now they are getting sick."
