But his top challenger, leftist candidate Andres Manuel Lopez Obrador, refused to concede, saying he would await a full count and legal review. He won roughly 31 percent of the vote, according to the preliminary count which has a margin of error of 1 percentage point. Lopez Obrador in 2006 paralyzed Mexico City streets with hundreds of thousands of supporters when he narrowly lost to President Felipe Calderon.

This time, only about 700 gathered at his campaign rally and he canceled plans to proceed to the Zocalo, the main square he filled as recently as Wednesday.

"We have information that indicates something different from what they're saying officially," he said. "We're not going to act in an irresponsible manner."

The PRI for 71 years ruled as a single party known for coercion and corruption, but also for building Mexico's institutions and social services. It was often accused of stealing elections, most infamously the 1988 presidential vote.