Mexicans are angry at the rise in price of their staple food

The price of the flat corn bread, the main source of calories for many poor Mexicans, recently rose by over 400%.

President Felipe Calderon has said the government will clamp down on hoarding and speculation to ease the problem.

But some blame the rise on demand for corn to make environmentally-friendly biofuels in the United States.

Agreement ignored

The BBC's Duncan Kennedy in Mexico City says the protest was noisy, passionate and angry.

The recent tortilla price rises have been the worst in decades, sparking fears that some could face malnourishment.

For many of Mexico's poorest people the tortilla is a staple of their diet, with as much as a third of their wages being spent on the bread.

Mr Calderon has ordered his agriculture secretary to import corn to try to ease the problem.

And earlier this month he signed a pact with a number of business groups that they would cap the price of tortillas at 8.5 pesos (77 US cents) per kilogram, but many have chosen to ignore the agreement, which is not legally binding.

Under the 1994 North American Free Trade Agreement, Mexico used to get cheap corn imports from the US, but Mexico's Economy Minister Eduardo Sojo has said that with more US corn being diverted into ethanol production, supply is dwindling.