Mexico's political left has filed a motion to invalidate the July 1 presidential election and demand a new vote, saying it has ample proof the process was fraud-ridden.

The election was marred by vote buying and therefore there is no "certainty for any result nor for the electoral process as a whole," Andres Manuel Lopez Obrador, who was the candidate of a coalition of leftist parties led by the Party of the Democratic Revolution, or PRD, said in a press conference Thursday.

The Institutional Revolutionary Party, or PRI, declared last week by electoral authorities to be the winner of the balloting, bought some 5 million votes, Lopez Obrador said, adding that in "free elections" most of these citizens would not have voted for Enrique Peña Nieto, the PRI's standard-bearer and presumed president-elect.

Based on the final recount, Peña Nieto won the election with 38.21 percent of the vote compared to 31.59 percent for Lopez Obrador, a difference of roughly 3.3 million ballots.

The TEPJF electoral court will examine the complaints and must hand down a final decision on the election's validity prior to Sept. 6.

Lopez Obrador, who showed reporters some ballots that he said had been marked by the PRI prior to election day, urged Mexicans not to allow "the constitution to be violated with impunity" and said he will not allow "corruption to completely dominate national life."

The politician also said that next week he will unveil a "national plan to defend Mexico's democracy and dignity." He provided no specifics but said that his movement will under no circumstances resort to violence.

Lopez Obrador also refused to accept the results of the previous presidential election in 2006, when he lost by a razor-thin margin to Felipe Calderon of the National Action Party, or PAN.

He said that year's balloting also had been marred by fraud, declared himself to be Mexico's "legitimate president" and organized a series of marches over several weeks in the capital's Paseo de la Reforma, one of Mexico City's main thoroughfares.

"Everything we'll do," he said Thursday, "will strictly adhere to our constitutionally guaranteed rights as citizens. I reiterate that we'll always act in a peaceful manner. We won't give the violent ones any pretext for accusing us of being violent."

Lopez Obrador said the challenge filed is based on article 41 of the Mexican constitution, "which establishes that elections must be free and fair."

The evidence to be examined by the electoral court was submitted Thursday to the Federal Electoral Institute, or IFE. It was transported in five vehicles and includes testimonials, videos, notarial acts and pre-paid cards supposedly given out to people in poor neighborhoods by the PRI.

Before handing over the evidence, representatives of the left appeared before reporters at the IFE headquarters in Mexico City surrounded by T-shirts, other campaign paraphernalia and boxes filled with documents to back up the fraud claims.

"We're convinced that based on the pile of evidence we're providing (and) the forcefulness of our arguments the tribunal should invalidate this electoral process and hold extraordinary elections," Jaime Cardenas, an attorney with Lopez Obrador's legal team, said.

Lopez Obrador's coalition alleges the PRI breached campaign spending limits and wrongfully acquired radio and television time to broadcast poll results.

They also manipulated those voter-preference surveys to demoralize supporters of other parties, engaged in vote-buying and coercion and set up parallel and illegal financing structures, the leftist coalition says.

Minutes after Lopez Obrador's appearance, PRI chairman Pedro Joaquin Coldwell told reporters the July 1 balloting was the "fairest election" in Mexican history and accused Lopez Obrador of being a "sore loser."

"The political preferences of millions of voters cannot be invalidated by an attitude that refused to acknowledge the legal truth and Mexico's political reality," Coldwell said.

The party chairman added that the PRI will go to the TEPJF "not only to defend the legality of this electoral process and its "legitimate victory" but also to defend the vote of millions of Mexicans who exercised a right that Lopez Obrador "wants to take away from them."

"The only problem with this election was having someone who has repeatedly proven to be a sore loser," Coldwell said.

In addition to Lopez Obrador, a student movement that arose a few months ago to protest alleged slanted media coverage in favor of the PRI during the campaign also has denounced irregularities during the process.

The most common reported irregularities the "Yo Soy 132" student movement compiled include the buying of votes and voter credentials and violations of a ban on campaigning in the days leading up to the election.

The PAN also said the election was marred by vote buying but that the irregularities were not sufficient to demand the results be invalidated.

If its victory is ratified by the TEPJF, the PRI, which governed Mexico uninterruptedly from 1929 to 2000, will return to the presidency in December after 12 years of PAN rule.

During its 71 years of largely unchallenged hegemony, the PRI relied mainly on patronage and control of organized labor and the mass media, though it was not above resorting to outright vote-rigging and even violence.

Although many Mexicans remain suspicious of the party due to its corrupt past, the PRI was able to regain power in large part due to spiraling drug-related violence that has left more than 50,000 dead during the presidency of Calderon, who took office in late 2006. EFE