Farmers drive tractors in downtown Mexico City Thursday during a protest against the removal of import tariffs on farm goods from U.S. and Canada, as agreed under the North American Free Trade Agreement. ((Eduardo Verdugo/Associated Press))

Led by a column of tractors, tens of thousands of demonstrators marched through downtown Mexico City on Thursday to protest recent trade openings that removed the last tariff protections for ancestral Mexican crops like corn and beans.

Chanting "Without corn, the country doesn't exist!" farmers and farm activists from across the nation demanded the Mexican government renegotiate the 1994 North American Free Trade Agreement, or NAFTA, to reinstate protection for basic crops.

Mexican farmers say the can't compete with bigger farms in the United States that receive more government support. Under the terms of NAFTA, Mexico got a 15-year protection period to improve its farms, but that phase-in period ended Jan. 1, and Mexican farms — mostly tiny plots of 12 acres or less — still lag behind.

"The truth is, we can't compete, that is why we're demonstrating ... because we're really getting hit hard," said Telespor Andrade, 44, a weather-beaten farmer from central Mexico who grows corn and beans on about seven acres of land.

Protesters pastured their cows outside the Mexican Stock Exchange on the city's main boulevard, and burned a tractor at a nearby monument to the 1910 Mexican Revolution.

Mexican officials say farmers are getting help, and that Mexico's corn production is rising. But activists say farm policies have benefited mainly big producers, not small producers who make up the vast majority of farmers. U.S. farmers, they say, have much better transport and distribution systems, lower costs and bigger subsidies.

"We're up against all the might of the developed countries," said Martin Perez Santiago, 60, a farmer from the Gulf coast state of Veracruz. "We can't compete because of a lack of support, a lack of subsides, technology, better seed varieties."

U.S. Ambassador Tony Garza released a statement on Thursday defending the trade pact, saying NAFTA's success has inspired other free trade agreements throughout the Western Hemisphere.

"NAFTA is the best example of the positive effects of free trade," Garza wrote. "For example, according to the U.N., chicken consumption in Mexico more than doubled in the last decade."

However, activists note that much of that chicken — and much of Mexico's livestock — is fed with imported U.S. corn.

Jose Ambris Juarez, a farm activist from Baja, Calif., said the country's farm problems were likely to fuel continued immigration to the United States.

"If the government doesn't help us find a way to make a living, what are we going to do? Just keep trying to cross the [border] line," he said.