Tens of thousands of Mexicans were trapped on rooftops and others clung to lampposts after heavy rains flooded nearly the entire southern state of Tabasco.
At least 500,000 people were made homeless and one person was killed in the worst flooding the swampy state has seen in more than 50 years.
President Felipe Calderon said it was one of the worst natural disasters in Mexico's history.
Television images showed rescue workers hauling people out turbulent, brown waters that rose as high as the roofs of houses. Children floated down a street in a plastic tub.
"It is getting very serious," Calderon said in a televised address.
Tabasco Governor Andres Granier said more than 1 million people, about half of the state's population, were "in the water," and scores of people called local radio programs pleading to be rescued.
Floodwaters turned many towns and swaths of the state capital, Villahermosa, into murky lakes.
Navy sailors and emergency workers steered boats and skiffs up and down streets, fishing thousands from the waters and dropping them off in dance halls, parking garages and other improvised shelters.