|Tractors were burned during the protest [Reuters]|
Thousands of Mexican farmers, many riding tractors and herding cows, have marched through Mexico City to demand government protection against cheap US agricultural imports.
Trade barriers were lifted in January under the North American Free Trade Agreement (Nafta), opening Mexico up for the first time to tax-free US exports of traditional food like corn and beans.
"The free trade agreement is like an open wound for the Mexican countryside," said Victor Suarez, the head of a small farmers' group.
"You can give the patient medical attention, but if you don't stop the hemorrhage first the patient will die."
Mexican farmers complain that the government of Felipe Calderon, the president, is not doing enough to protect them against heavily subsidised US goods and are demanding that Mexico renegotiate the treaty.
Since Nafta came in to force in 1994, corn tariffs have gradually been phased out and imports of US yellow corn to Mexico, mostly used in animal feed, have increased.
They now account for close to 35 per cent of Mexican consumption.
Farmers set one tractor on fire and built an enclosure for dairy cows in front of the Mexican stock exchange.
Some carried black crosses or coffins representing the death of rural Mexico.
The farmers fear the absence of tariffs will encourage large US farms to start producing white corn, which has been a major part of the Mexican diet since the Aztec era.
Opposition politicians have called for the resignation of Alberto Cardenas, the agriculture minister, for failing to do enough to support farmers.
Cardenas said on Wednesday the government would offer support to farmers to buy corn for animal feed, since international prices have rocketed in recent months.
The minister said the negative effects of the trade deal for corn and wheat growers would be offset by high international prices caused by increasing US demand for ethanol, which is made from corn.