Rescue workers in southern Mexico are battling to help some 300,000 people trapped in their homes by flooding, with more rain forecast in coming days.
The state of Tabasco is suffering its worst flooding in more than 50 years, with most of the state under water, 1m people affected and at least one death.
The state governor has urged anyone who owns a boat to help the rescue efforts, alongside army and navy crews.
President Felipe Calderon said the situation was "extraordinarily grave".
Red Cross officials said 80% of the state was flooded and the waters were still rising.
Rescuers are using helicopters to try to pluck people from rooftops. Thousands of people are huddled inside their homes or in emergency shelters.
Mr Calderon called the flooding "one of the worst natural disasters in the history of the country" and appealed for Mexicans to help by donating supplies.
In a televised address on Thursday night he said many people had "lost their homes, their belongings, their crops and the means to maintain their children".
He added: "Others remain in their homes but with no access to food, water or medicine."
The floods were triggered by storms that crippled Mexico's oil industry.
Tabasco Governor Andres Granier said more than half of the state's 2.1 million residents had been affected and the state was "devastated" with 100% of the crops lost.
He drew a comparison to the damage caused by Hurricane Katrina on the US Gulf Coast two years ago, saying: "New Orleans was small compared to this."
The state has been placed on high alert, with forecasters predicting that a new cold front could bring more rain over the weekend.
A spokesman from Tabasco's ministry of health said there was a shortage of clean water and food.
The spokesman, Dr Garcia Rochin, said "the situation is very, very difficult.
"Our shelters are packed with people, and we are still running rescues in the flooded areas using helicopters and boats," he told the BBC.
Red Cross official Max Romero said 650 shelters had been set up on high ground in Tabasco and in neighbouring Campeche and Veracruz states.
Tabasco's capital, Villahermosa, and many other towns in the state have been turned into brown lakes with only treetops and roofs visible.
The capital's historic city centre was evacuated late Thursday as the Grijalva river burst its banks.
A Red Cross worker in Tabasco state said 70% of Villahermosa was affected and there was an urgent need for basic materials to help the rescue effort.
Those trying to help flood victims needed water supplies, food and mattresses, he told the BBC.
An appeal was being made for boats and ropes, to help navigate the flooded streets and reach stranded victims.
The main priority for rescuers was to try and get all the affected people out before more rains arrived, he added.
Sandbags were placed around several giant heads carved by the Olmecs, an ancient pre-Columbian people, at Tabasco's La Venta archaeological site, but the water rose half-way up the three-metre tall statues.
Flooding has also affected the southern state of Chiapas, where several thousand people have been moved to safety.
Twenty-one people died last week when storms forced an oil platform into another rig in the Gulf of Mexico.
The storms have forced the closure of three of Mexico's main oil ports, preventing almost all exports and halting a fifth of the country's oil production.
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