Hundreds of thousands of people still in mortal danger have been stranded by intense flooding in many parts of Mexico.
President Felipe Calderon said: "The situation is extraordinarily grave: This is one of the worst natural disasters in the history of the country."
He appealed for Mexicans to donate supplies, saying many people had "lost their homes, belongings, crops and the means to maintain their children". Others remained in their homes but with no access to food, water or medicine, he added.
A week of heavy rains caused rivers to burst their banks, leaving 70 per cent of the southern state of Tabasco inundated and forcing tens of thousands of people to clamber on to rooftops to escape the rising waters.
The floods have endangered more than one million residents, or about half Tabasco's population. Parts of Villahermosa, the state capital, and many other towns and villages have been turned into ugly brown lakes with only treetops and roofs visible above the waterline.
Some 400 doctors and health workers were deployed to more than 300 towns in the region to detect any outbreak of infections.
The floods began last week when a cold front brought heavy rain, causing the Grijalva, Carrizal and Puxcatan rivers to burst their banks. The storms have also crippled Mexico's oil industry.
All day yesterday, soldiers and rescuers desperately stacked sandbags along Villahermosa's streets. Sandbags were also placed around several giant ceremonial heads carved by the Olmecs, an ancient pre-Columbian people, at Tabasco's La Venta archaeological site. A Red Cross worker in Tabasco state saidthat 70 per cent of Villahermosa was affected and there was an urgent need for basic materials to help the rescue effort.
Tabasco's governor, Andres Granier, said that more than half of the state's 2.1 million residents had been affected and the state was "devastated".
"We have lost 100 per cent of our crops and 70 per cent of the state is under water," he told reporters. "New Orleans was small compared to this," in reference to the flooding caused by Hurricane Katrina in 2005, which killed about 1,000 people.
Weather forecasters are predicting more rain and misery in the coming days as a new cold front moves in. In addition, the Grijalva River, which circles the city of Villahermosa, was pouring into the streets after bursting its banks and rising 6.5ft above its "critical" level.
Boats and helicopters being used to remove residents from rooftops and homes were overwhelmed by the demands to take thousands of people to shelters.
An estimated 300,000 were still awaiting rescue yesterday after their homes were flooded. There were no more supplies of drinking water in Villahermosa, and soldiers were still trying to reach those still in danger.
Outside government offices, dozens of survivors who were anxious about relatives and friends crowded around seeking information from anyone who knew anything. Others were seen wading despondently through waist-deep water or walking along highways out of the state capital.
"We lost everything," said Manuel Gonzalez, whose house was swallowed by the waters. "I left without one peso in my pocket and I can't find my siblings."
In the sometimes-troubled state of Chiapas, the serious flooding has hugely affected more than 100,000 of its residents. President Calderon has asked Mexicans to contribute bottled water, canned goods, nappies and other vital supplies to donation centres around the country.
"Nobody can stand around with his arms crossed," he said. "We can't and won't abandon our brothers and sisters in Tabasco."