As many as 12,500 people answered the call of leftist opposition leader Andres Lopez Obrador to march peacefully against the plans, a public safety official said yesterday.
"They want to share out with foreign oil companies, the earnings of the people, the nation," said Lopez Obrador, charging the plan was an affront to Mexican sovereignty.
He called on supporters to reject energy industry reforms, unless they are put before the people in a referendum vote.
State oil giant Pemex generates more than a third of the money that funds the government's budget.
Its output has slid from 3.4 million barrels per day in 2004 to 2.5 million in 2012.
The government wants to raise money with foreign partners for high-cost, super-deep oil exploration beneath the Gulf of Mexico.
