Tens of thousands of people marched in Mexico City to protest constitutional reforms pushed through by President Enrique Pena Nieto that open the oil and gas industry to foreign investment.
An estimated 65,000 people on Friday gathered for the protest in the Zocalo, the main square in capital city, an official at the Secretariat of Public Safety told AFP.
Some 2,500 police officers were deployed but there were no incidents of violence, the official said.
The march was organized by the Party of the Democratic Revolution (PRD), the leftist opposition to the president's ruling Institutional Revolutionary Party (PRI).
The reforms, which open Mexico's oil industry to foreign investment for the first time in 75 years, were approved in Congress and ratified by a majority of Mexican states in late 2013.
The rule changes are supported by two of the country's leading parties, the PRI and the conservative National Action Party (PAN).
But the third party, the PRD, vehemently opposes the reforms.
Many in Mexico look back with pride at the expulsions of foreign oil companies in 1938 by then president Lazaro Cardenas.
One of the PRD's founders is the late leader's son, Cuauhtemoc Cardenas.
"All types of protest are valid" in opposing the reforms, Cardenas told the crowd in the Zocalo, "including civil disobedience."
The foreign investors "will be interested in extracting the largest amount of petroleum possible in the shortest amount of time," Cardenas claimed.
The PRD is hoping to hold a referendum in 2015 to overturn the measures.