A single poster is the only reminder that Mexico is celebrating its bicentenary this week in a former colonial port and UNESCO heritage site in flood-hit southeastern Mexico.

The brightly colored buildings of Tlacotalpan have been under water for several weeks, part of swathes of Mexico soaked by the worst rainy season on record.

Almost one million people were affected by flooding this month alone which left 25 dead. The rains, which began in July, are set to worsen as the season continues to almost the end of the year.

The mayor of Tlacotalpan on Monday called on the United Nations to help evaluate damage to 540 flooded buildings which earned it classification by UNESCO, as residents sought to pick up their lives.

Antonio Cruz searched for food for his children as they played in a nearby puddle.

A poster carrying the red, white and green of the Mexican flag in the Colonial cafe, calling on Mexicans to "celebrate together," survived flooding which forced the evacuation of most of the town's 8,500 inhabitants.

"They tell us that the governor will come to celebrate, and that they'll distract us a bit from all the water," said 38-year-old fisherman Cruz.

His furniture -- two beds, a wardrobe and a few electrical items -- hung from the ceiling of his house, above one meter (yard) of water.

The Papaloapan river swept through the town at night, sweeping some residents with it, Cruz said.

"Luckily no one drowned," he added.

More than one third of the Gulf Coast state of Veracruz has been hit by flooding which affected some 500,000 people, according to governor Fidel Herrera.

Around 25,000 flood victims would spend September 15-16 in makeshift accommodation across the state while elsewhere, particularly in the capital, many prepared for a massive party to celebrate 200 years of independence.

The bicentennial pays homage to the 1810 uprising began by Catholic priest Miguel Hidalgo, which led to the ousting of the Spanish by 1821.

It comes not only amid flooding, but also a wave of drug-related violence which has left more than 28,000 dead since 2006, according to official figures.

A string of towns and cities worst affected by the violence have also scaled down their bicentennial ceremonies.

In Tlacotalpan, several dozen neighbors prepared for modest celebrations in a refuge on the second floor of a local school.

"It's ironic that we have so much water around us, and we don't have clean water for washing," said Tomasa, a grandmother who said she could not remember such a wet independence day.

After the closure of two dam gates, the river level dropped and parts of the historic town, such as the main square, started to emerge.

Marines cleaned off the mud ahead of Wednesday's traditional shout of "El Grito" -- a reconstruction of Hidalgo's 1810 battle cry.

It would take place this year by day instead of night to make it easier for the governor and some of the 1,000 people remaining to attend, officials said.

On a street corner, a partly-submerged inscription read: "I'll never leave my town."