The threat has loomed large over this giant metropolis for weeks: Floodwaters could rapidly swamp glitzy downtown Bangkok, ruining treasured ancient palaces and chic boutiques on skyscraper-lined avenues in the heart of the Thai economy.

The floods haven't come, but the sense of imminent doom is growing by the day, seeping in through worried conversations, school closings and emptied store shelves. One measure of the fear: the protective walls of sandbags scattered across the city's canals, homes and shop-fronts are expanding in number and height daily.

Many Bangkokians are girding for the worst after a week of mixed government messages that have failed to answer their most pressing question: Will the capital succumb to the worst floods to strike Thailand in half a century?

"The water is coming, it's inevitable," said Oraphin Jungkasemsuk, a 40-year-old employee of Bangkok Bank's headquarters. Its outer wall is protected by a six-foot-high (two-meter-high) wall of sandbags wrapped in thin plastic sheeting.

"They are fighting a massive pool of water. They cannot control it anymore," Oraphin said. "There are barriers, but it can come into the city from any direction, even up through the drains."

A long season of monsoon rains and storms has wasted a vast swath of Asia this year, killing 745 people _ a quarter of them children _ in Thailand, Cambodia, Vietnam, Laos and the Philippines, according to the United Nations.

Thailand's government says at least 320 of those deaths occurred here, mostly from drowning as floodwaters crept across this Southeast Asian nation since July, submerging whole towns under water more than six-feet-high (two-meters-high) and damaging as much as one-tenth of rice fields in the world's top rice exporter.

About two weeks ago, the capital itself began waking up to the reality of potential catastrophe as floods dramatically overwhelmed neighboring provinces. The drama has fueled panicked exoduses from the hardest-hit areas and, in Bangkok, shopping sprees as residents stocked up on emergency supplies.

Prime Minister Yingluck Shinawatra on Wednesday acknowledged the crisis has overwhelmed her nascent government. On Thursday, she announced authorities were opening floodgates that had been keeping water out of the city. It's an attempt to let the vast flood pools empty into the sea, but the move risks a potential overflow as water runs through already inundated canals.

"We must allow the water to flow through. Very little has been driven to the sea," Yingluck said. She added: the water is "all over the place and has nowhere to go."

Much is at stake. Economic analysts say the floods have already cut Thailand's 2011 GDP projections by as much as 2 percent. Damages could run as high as $6 billion _ an amount that could double if floods swamp Bangkok.

This week, Bangkok's governor called for 1 million sandbags to reinforce vulnerable spots _ on top of 1 million more called for earlier this month. The Thai military and volunteers have been bolstering flood walls that ring Bangkok for miles (kilometers), many of them along a complex network of swamped canals.

Record water levels have already sent water spilling over the city's main Prapa canal, spattering the streets Bangkok. There were no immediate major problems late Thursday, but the city's governor urged residents to remain alert.

Still, there is plenty to worry about. Authorities have urged residents in seven of the city's northern districts to prepare for inundation and move belongings to higher ground.

On Thursday, vehicles began parking on elevated expressway bridges on the northern outskirts of the city, snarling traffic as trucks piled high with people apparently fleeing flood-hit zones moved through the streets.

But much of downtown looked totally normal, if eerily calm, and a quiet panic was palpable.

Oraphin Milintanon, who works at a camera shop in the capital where customers must step across sandbags to get inside, has watched the floods advance with increasing alarm.

The water first swept through her hometown in the now-heavily submerged city of Ayutthaya, just north of Bangkok. Then it poured through her current home in Nonthaburi province.

Oraphin now lives with a sister in a dry part of Bangkok, but tales of water creeping closer are spooking residents. She said her brother, living elsewhere in Nonthaburi, was recently awaken by the flood water itself _ which welled up suddenly into his home as he slept on his bed.

"It can come very fast ... the problem is, nobody knows from where it will come," Oraphin said. The only thing certain, she added, "We know it is coming soon."

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Associated Press writers Vee Intarakratug, Thanyarat Doksone and Grant Peck contributed to this report.