Mexican officials provided flooded Gulf coast with drinking water, food and clothing as rescuers in boats and helicopters tried to save thousands of people.
With floodwaters stretching clear across the Gulf coast state of Tabasco and food and drinking water scarce, health officials warned against cholera and other waterborne diseases.
An estimated 900,000 people had their homes flooded, damaged or cut off from access. Tabasco Gov. Andres Granier said 300,000 were awaiting rescue Thursday, and police, soldiers and military workers were still trying to reach them.
Soldiers evacuated the state capital of Villahermosa's historic center Thursday night as the waters of the Grijalva river burst through dikes of sandbags and toppled a retention wall, flooding the city's bus station and open-air market. Much of the city looked like New Orleans after Hurricane Katrina, with murky water reaching to rooftops.
Safe refuges were scarce and officials improvised, turning parking garages and any other dry structure into temporary shelters.
Dozens of hospitals and medical centers were flooded. Health officials warned that there could be epidemics of cholera, although none was reported.
Tabasco state floods every year around this time, and many poor, low-lying neighborhoods have grown accustomed to spending half a year with the first floor of their homes under water. But this year has taken even flood-weary residents by surprise, inundating Villahermosa and leaving the city's famous Olmec statues with water up to their enormous stone chins.
"The situation is extraordinarily grave: This is one of the worst natural disasters in the history of the country," President Felipe Calderon said in a televised address Thursday night.
Several Mexican banks established special accounts for donations to flood victims, many of whom lost everything, including their homes.
"Nobody can stand around with his arms crossed," Calderon said. "We can't and won't abandon our brothers and sisters in Tabasco."
A week of heavy rains caused area rivers to overflow, leaving at least 70 percent of the state and 80 percent of the capital under water. At least one death was reported. Nearly all services, including drinking water and public transportation, were shut down in Villahermosa.
Weather forecasters predicted more rain in the coming days. The flooding was not related to Tropical Storm Noel, which pounded the Caribbean.
The Grijalva River, one of two large waterways ringing Villahermosa, has risen 2 meters (6.5 feet) above its "critical" level and gushed into the city's center. Authorities said some of the rivers were continuing to rise.
In Villahermosa, dozens of survivors anxious about relatives and friends crowded outside government offices seeking assistance. Others waded despondently through waist-deep water or wandered along highways leading out of the capital.
"We lost everything," said Manuel Gonzalez, whose house was swallowed by the floodwaters. "I left without one peso in my pocket and I can't find my siblings."
The state of Chiapas, which borders Tabasco to the south, also reported severe flooding, with officials there estimating that more than 100,000 people have been affected.