By Alberto Fajardo

VILLAHERMOSA, Mexico, Nov 3 (Reuters) - Thousands of people perched on rooftops in southern Mexico on Saturday, hoping to be evacuated from devastating flooding that has left most of Tabasco state under water and 800,000 people homeless.

Some 70,000 people spent the night in emergency shelters, as the army brought in food and drinking water and airlifted people out by helicopter or took them out by boat. But there was only one death blamed on the disaster.

"We need help," one woman told Reuters Television after being rescued by helicopter from the roof of a school.

"There are a lot of people up there, there are pregnant women, children. They didn't want to leave their homes but there's now no other option. We've lost everything."

With many stores in the area running out of provisions, several thousand others opted to flee on buses on Friday to the neighboring states of Veracruz and Campeche.

Hopes that floodwaters may be receding thanks to sunny weather on Friday were dimmer after the night brought more rains that left the water flowing even faster. Power was out and fixed-line and cellular telephone networks were also down.

People and livestock swam through streets in neck-high murky brown water on Friday after floodwaters burst through sandbag barriers into the state capital, Villahermosa, home to about half a million people.

The Grijalva River that winds through Villahermosa swelled over its banks earlier in the week after days of heavy rains that triggered some of the worst flooding the swampy, low-lying region has seen in half a century, killing one person.

President Felipe Calderon called the situation "critical" as flood levels reached 19 feet (6 metres) in some areas, and food supplies were trucked in from across the country.

The stormy weather also disrupted Mexico's oil shipments to the United States for most of the week, although state energy monopoly Pemex reported no impact on its oil wells in Tabasco.

Gov. Andres Granier said more than 1 million people, about half of the state's population, were affected by the flooding.

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