Hundreds of thousands of people have fled severe floods in the south Mexican state of Tabasco where rivers burst their banks after heavy rain.
Mexican soldiers have been deployed to keep order
The centre of the state capital, Villahermosa, is under between 2m (6 feet) and 6m of water with only rooftops visible from the air.
Some 300,000 are still trapped in their homes in Tabasco, waiting to be rescued by boat or helicopter.
Overall, the floods have claimed at least five lives in southern Mexico.
Officially, one person has died in Tabasco due to the floods, and four people were killed in the neighbouring state of Chiapas, where several thousand homes have been affected.
President Felipe Calderon has ordered the entire air force to help bring supplies into the region and move people out.
Villahermosa is completely overwhelmed by murky, filthy water, the BBC's Andy Gallacher reports from the city.
Soldiers, first-aid workers and volunteers are scouring the streets in small boats, handing out fresh food and water, both of which are now in short supply.
The Mexican government has admitted that the scale of this disaster has taken them by surprise and for those still trapped in the remote areas of Tabasco aid is still a long way off.
As the massive operation continues, there are now fears that disease could spread and forecasters say more heavy rain is on the way.
Many people left in Villahermosa seem resigned to spending another night on their rooftops while tens of thousands are crammed into emergency shelters struggling to provide enough hot meals and dry beds.
One group spotted by a Reuters reporter on a roof held a banner reading: "There are children, pregnant women, sick women. Send the police."
A woman rescued by helicopter from the roof of a school said those left behind included "pregnant women and children".
"They didn't want to leave their homes but there's now no other option - we've lost everything," she told Reuters.
President Calderon has given special powers to soldiers and police to maintain order and prevent looting and called on private airlines to help with the relief efforts as most roads in and out of the state have been cut off by the floodwaters.
After flying over the state, Mr Calderon described the flooding as "not just the worst natural catastrophe in the state's history but, I would venture to say, one of the worst in the country's recent history".
Tabasco state governor Andres Granier said the floods were a "monster" and urged anyone who owned a boat to help out.
The floods were triggered by storms that crippled Mexico's oil industry.
Twenty-one people died last week when storms forced an oil platform into another rig in the Gulf of Mexico.
Flooding has also affected the southern state of Chiapas, where several thousand people have been moved to safety.
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.