MEXICO CITY -- Thousands of farmers on foot and on lumbering tractors clogged Mexico City Thursday to protest the lifting of corn tariffs under a free trade agreement, which they say is hurting their pockets.

"No corn, no country" was the byword of the protest plastered in signs on tractors and buses, as the angry farmers, some of them leading herds of cattle through the streets, demanded equal treatment with farmers in the United States and Canada.

While it was mostly peaceful, there was some tension late Wednesday when a column of slow-moving tractors ground to a reluctant halt before a phalanx of anti-riot police that barred access to the Zocalo, the city's main square.

Some 1,500 police have fanned out across the city to prevent any unrest stemming from the protest, as farmers from across the country have made their way here, some on foot for 1,600 kilometers (1,000 miles), since January 18.

A provision of the North American Free Trade Agreement (NAFTA) lifting tariffs on corn -- Mexico's staple food -- kicked in on January 1, 14 years after the agreement between the three neighbors came into being.

Many farmers in Mexico have been against NAFTA from the start, but their protest has escalated as the date for lifting corn tariffs approached.

The National Peasant Confederation (CNC), Mexico's chief farmers' union with more than five million members, has also warned against NAFTA regulations lifting tariffs on milk and sugar cane products.

Farmers also say that government subsidies their counterparts in Canada and United States receive are unfair. CNC said farmers get some $20,000 in annual subsidies in the United States compared to only $700 in Mexico.

They also complain of mounting fuel, fertilizer and electricity prices which they claim represent 60 percent of the average cost of running a farm and place them at a severe disadvantage to their northern competitors.

The farmers and opposition politicians are insisting that some NAFTA provisions be renegotiated, but the three NAFTA governments refuse to do so.

Canada's Agriculture Minister Gerry Ritz last week said the United States, Mexico and Canada were pleased at how NAFTA was working and saw no reason to reopen negotiations.

Leftist opposition lawmaker Victor Quintana, however, asked that "at least corn and beans be removed" from the list of products allowed tariff-free into Mexico.

The NAFTA agremement, he told reporters, "is a disaster for Mexican farmers, for the people's food security, for national security and for the country's democratic rule."

For University of Chapingo agronomist Rita Schwentesius, the 1,000 farm products exempt of tariffs since January 1 "will have no ecomomic impact. There will be no crisis because grain prices (corn and beans included) have gone up on the international market."

But corn grower Luis Valdiga, 49, who drove his tractor here from central Aguascalientes state, saw things differently.

"Before NAFTA, we could live off our crops. Now they're worthless. What can we do?"