As many as half a million people have seen their homes damaged or destroyed by massive floods in southern Mexico, authorities say.
Officials are struggling to provide food and drinking water
The military has helped to evacuate thousands of people in the states of Tabasco and Chiapas. Overall the floods have claimed at least five lives.
Officials say the floodwaters have begun to recede in Tabasco but thousands of people remain homeless.
Others are still trapped in their homes or stranded in isolated communities.
Only one death has been reported in Tabasco, although in the poor southern state of Chiapas, local government officials reported four deaths on Sunday after rain-swollen rivers burst their banks, damaging thousands of homes and 16 bridges.
Most of the capital of Tabasco, Villahermosa, has now been evacuated and resources are being concentrated in evacuation centres, says the BBC's Andy Gallacher.
Tensions have been rising and there have been some reports of looting as supplies of food and drinking water dwindle.
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.
President Calderon has given special powers to soldiers and police to maintain order and called on private airlines to help with the relief efforts.
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".
The floods were triggered by storms that also 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.