Thousands of Mexicans took to the streets of Mexico City to protest an energy reform proposal that would allow more private investment in the country's oil industry.

Those taking part in Sunday's protest included former presidential candidate Andres Manuel Lopez Obrador, who was quoted by the French news agency AFP as saying the plan by President Felipe Calderon violates "the spirit and letter" of Mexico's constitution.

Earlier this month, lawmakers allied with Lopez Obrador camped out in the legislative chambers in an effort to delay approval of the legislation.

The bill would give state-owned oil company Petroleos Mexicanos (Pemex) more flexibility to hire private companies to help with refining, deep water exploration and production.

Opponents say opening the industry to private investment would threaten Mexico's sovereignty.

Mexico's constitution has severely restricted private investment in energy since 1938. Mexico is a major supplier to the United States.