Tens of thousands of Bangkok residents jammed bus stations and highways Wednesday to flee Thailand's flood-threatened capital as the city's governor ordered official evacuations in two swamped northern districts for the first time since the crisis began.

Floodwaters bearing down on the metropolis have killed 373 people nationwide since July, causing billions of dollars in damage and shutting down Bangkok's second largest airport.

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People play in a flooded temple in Bangkok on Wednesday. Fears of flooding across the Thai capital grew after Prime Minister Yingluck Shinawatra warned parts of Bangkok could face inundation for up to a month. ((Sukree Sukplang/Reuters))

Prime Minister Yingluck Shinawatra's government had repeatedly vowed to protect the capital, which has so far mostly escaped unscathed. But official assessments have turned grim in recent days, and everywhere people are preparing for flooding that seems all but inevitable.

Gov. Sukhumbhand Paribatra said residents of two of the city's 50 districts — Don Muang and Bang Phlat, both already partially submerged -- should leave for safer city shelters.

"This is the first time I am using the term 'evacuation,' the first time I'm really asking you to leave," Sukhumbhand said.

Elsewhere in the city, thousands of people packed Bangkok's Mo Chit bus terminal, trying to leave town on their own. Many appeared to be taking advantage of a government-declared five-day public holiday to avoid a possible watery siege. The holiday runs Thursday through Monday in flood-affected areas, including Bangkok.

Some waited for hours on the sidewalk outside Mo Chit because there was no space inside the terminal, the main departure point for buses to Thailand's north.

The mass exodus included thousands of migrants from neighbouring Myanmar, workers dependent on low-paying jobs so desperate to leave they are willing to brave a return to their intensely repressive nation to do so.

Authorities were also forced to move hundreds of inmates from three prisons — many on death row — to facilities in other provinces.

Satellite maps of Bangkok showed a city almost entirely surrounded by water. Most of the vast pools of runoff now submerging a third of the country are flowing from the north toward Bangkok — southward toward the Gulf of Thailand.

"The amount of water is gigantic," Yingluck said. "Some water must spread into Bangkok areas but we will try to make it pass through as quickly as possible."

Young drowing victims

Children make up around a quarter of the nearly 800 deaths reported since July across Thailand, Cambodia, Vietnam, Laos and the Philippines, according to the United Nations. The region has been ravaged by some of the worst flooding in decades, but drownings are a huge unreported epidemic in Asia. Every year, an estimated 240,000 children up to 17 years old die -- mostly because the majority of kids simply never learn to swim.

That annual number is roughly equal to the total deaths from the 2004 Indian Ocean tsunami, but day-to-day water deaths rarely get attention.

"Those (in the tsunami) were counted because they drowned in a space of six to eight hours in the region, and everyone was just stunned because the number was enormous," said Michael Linnan, technical director of the U.S.-based Alliance for Safe Children in Bangkok, who has studied child drowning.

"But the reality is that in that the 364 days before that, an equal number of mothers and children had drowned as well. But they drown one at a time and not in a disaster setting, so they weren't counted."

— The Associated Press

In the district of Sai Mai, located on the capital's northern outskirts, waist-high water turned roads into virtual rivers and swamped gas stations and homes.

Hundreds of residents clamoured aboard packed military trucks with their belongings, desperate to leave. But help was in short supply.

"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 got out any way they could -- in paddle boats, plastic tubs, inner tubes and rubber rafts. Several men floated down a flooded road in a makeshift boat made of empty oil barrels tied to a rectangular plank.

As fears of urban disaster set in, some residents built cement walls to protect their shops and homes.

Websites posted instructions on the proper way to stack sandbags. Many residents fortified vulnerable areas of their houses with bricks, gypsum board and plastic sheets. Walls of sandbags or cinderblocks covered the entrances of many buildings.

Concern that pumps would fail prompted a run on plastic containers in which to hoard water. Anticipating worse, one woman travelling on Bangkok's Skytrain system carried a bag of life vests.

On Tuesday, floods breached barriers protecting the capital's Don Muang airport, shutting it down hours later. The airport is used primarily for domestic flights, but its closure dealt a major psychological blow to efforts to protect the capital.

The country's main international airport is still functioning normally.

Panic has gripped parts of the city as more and more of it is affected by the advancing water. Residents stocking up on food and other necessities have emptied supermarket shelves. Bottled water and toilet paper were in especially short supply.

Yingluck urged everyone in the capital to move their belongings to higher ground and warned that the city's fate rested on three key flood barriers.