Up to 65,000 people marched on Mexico City Friday to protest changes in the country’s oil and gas laws.
The crowd gathered in Zocalo square.
The leftist Party of the Democratic Revolution (PRD) organized the demonstration.
New laws enacted late last year allow foreign investment and private ownership in the oil and gas sector for the first time in 75 years.
The protestors allege the reforms will lead to asset stripping by private interests and corrupt politicians.
President Enrique Pena Nieto proposed the reforms to revive Mexico’s flagging production.
The new laws were approved in Congress and ratified by a majority of Mexican states late last year.
Under the 1938 constitution, Pemex — the state-owned oil and gas company — was granted a monopoly over the oil and gas industry, and foreign capital was banned.
Mexico’s crude production has fallen to less than 2.5 million barrels per day from a high in 2004 of 3.4 million barrels per day.
At least 2,500 police officers were deployed to Zocalo square Friday but there were no reports of violence.
The protest was the biggest demonstration against the reforms since December.
A founder of the opposition PRD party is Cuauhtemoc Cardenas. His father was Mexico’s president in 1938 when foreign companies were expelled from the oil and gas sector.
“All types of protest are valid” in opposing the reforms, Cardenas told the crowd at Zocalo square Friday, “including civil disobedience.”
The PRD has said it wants a referendum in 2015 to repeal the reforms.