Mexico's electoral courts will be asked later this week to annul or declare the July 1 presidential election invalid on the grounds that 5 million votes "were bought" for the winning Institutional Revolutionary Party, or PRI, leftist presidential candidate Andres Manuel Lopez Obrador said Monday.

The motion will be filed on Thursday, Lopez Obrador, who finished second in the presidential race, said in a press conference.

PRI candidate Enrique Peña Nieto won Mexico's presidency with 38.21 percent of the vote, while Lopez Obrador took second place with 31.59 percent, according to the final official results released by the Federal Electoral Institute, or IFE.

The results are subject to challenge under Mexican elections law.

There is ample evidence that the constitutional guarantees of clean elections were violated, the leftist candidate said.

"What we have no doubt about is that around 5 million votes were bought" for Peña Nieto, Lopez Obrador said.

There are "two ways (to remedy the situation), annulment or declaring the elections invalid," Lopez Obrador, a former Mexico City mayor who was the standard-bearer of a leftist coalition led by the Party of the Democratic Revolution, or PRD, said.

"We are going to present the evidence on Thursday, and we are going to lay out the type of proceedings we want, of invalidation or of annulment," Lopez Obrador said.

Even though more than half of the ballots cast were recounted, "it is not enough" to ensure a clean election, the leftist politician said.

The recount was "very irregular, with the opening of (ballot) packages that had already been opened, ballots that arrived outside (the packages) and many other things," Lopez Obrador said.

"We are going to follow the legal process. Our movement has been, is and will continue to be peaceful. We are not going to compromise on the defense of democracy," Lopez Obrador, who ran under the banner of the Progressive Movement coalition, said.

Lopez Obrador lost the 2006 presidential election to Felipe Calderon, of the National Action Party, or PAN, by 0.56 percent and has never recognized the results, claiming victory in the contest.

The PRI, which governed Mexico from 1929 to 2000, lost the 2000 and 2006 presidential elections to the conservative PAN.

Peña Nieto will be inaugurated on Dec. 1. EFE