Thailand's prime minister warned that the kingdom would endure at least one more month of flooding, telling anxious residents in the capital Bangkok to prepare for possible meter-deep water.
The authorities have launched a high-stakes attempt to channel the floodwaters from the central plains out to sea through canals in the city, which has already seen waist-high water in parts of its northern outskirts.
Prime Minister Yingluck Shinawatra, who has invoked a disaster law to take full control of the emergency response, told Bangkok residents to move their belongings at least one metre (3.3 feet) off the ground.
"Bangkok must open all floodgates to allow the water through," said Yingluck. "So during this long weekend residents should move belongings, cars and other valuable things to places at least one metre high."
"There is a huge volume of run-off water from the north and we can't effectively block it but can only slow the flow because our barriers are temporary," she said in her televised weekly address to the nation.
"It's an extremely serious situation that affects people's lives and property," added Yingluck, whose two-month-old government is racing to avert a humanitarian disaster.
Three months of heavy monsoon rains have killed at least 356 people in Thailand and damaged the homes and livelihoods of nine million people, mostly in the north and centre, the government said.
About 113,000 people have been forced to seek refuge in shelters, Yingluck said, adding that while the waters were receding in some areas of the country, the floods heading towards the capital were unstoppable.
The overall flood situation would continue for "four to six weeks", she added.
In Bangkok, home to 12 million people, residents have rushed to stock up on food and bottled water, while motorists have parked hundreds of cars on bridges or elevated roads.
Tens of thousands of soldiers and police have been mobilized to maintain order.
The opposition is calling on the government to declare a state of emergency to make it easier to control people and protect flood barriers, but Yingluck has ruled out such a move.
She said the authorities would secure important locations such as the palaces, government buildings, major utilities and key transport routes.
"The government will oversee the distribution of all consumer goods and ensure that they are sufficient and available for everyone."
A political novice before taking office, the sister of fugitive former leader Thaksin Shinawatra is facing the first major test of her fledgling leadership.
The authorities have failed to protect a number of major industrial parks from the gushing brown water, which has inundated hundreds of factories outside Bangkok, disrupting the production of cars, electronics and other goods.
The government says more than half a million people have seen their jobs disappear for now.
Most of Thailand's main tourist attractions -- including the southern islands of Samui, Phuket and Phi Phi -- have been unaffected although some foreign governments have warned against non-essential travel to Bangkok.
Bangkok's main airport, built on a drained marsh, is still operating as normal and its flood defences have been reinforced.