Thailand's authorities are trying to channel the floodwaters through Bangkok canals in an effort to spare the city center as floods put more pressure on the barriers protecting the capital.
Some of the northern suburbs have already seen waist-high water. One third of the Asian country has been inundated after three months of rain.
Thai Prime Minister Yingluck Shinawatra went on national television Saturday to warn citizens that water levels would rise and that flooding could take as long as six weeks to subside.
She urged Bangkok residents to move their possessions to higher ground.
The worst flooding in 50 years in Thailand has killed 356 people and at least two more are reported missing. The rain has damaged rice crops, Thailand's staple food, and has forced closure of industrial plants. The labor ministry says 700,000 have been put out of work temporarily. More than 110,000 people have been displaced.
The Thai prime minister has also invoked the natural disaster law that gives her authority to implement a nationwide disaster relief plan and punish the officials who fail to follow instructions.
Heavy rains since July have inundated large parts of Southeast Asia, forcing millions from their homes. Another 336 people have died in Cambodia, Laos and Vietnam.
A growing number of countries are pledging aid to the flood-ravaged region.