(adds left's reaction to court ruling)

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The decision by Mexico's top electoral court to validate the victory of Institutional Revolutionary Party, or PRI, candidate Enrique Peña Nieto in the July 1 presidential election is an affront to the rule of law, the country's main leftist party said Friday.

"We maintain that the election did not adhere to the principles of legality, impartiality, equity, liberty, objectivity and certainty," PRD chairman Jesus Zambrano told reporters.

"To accept (the election outcome) ... would be to renounce the possibility and the necessity of ceasing to be the most discredited democracy in Latin America," he said.

The leader of the Party of the Democratic Revolution, or PRD, convened a press conference shortly after Mexico's TEPJF electoral tribunal voted unanimously to uphold the election results and designate Peña Nieto as president-elect.

The ruling came a day after the TEPJF dismissed a challenge filed by the leftist Progressive Movement coalition, whose candidate, Andres Manuel Lopez Obrador of the PRD, finished second in the balloting.

The judges found that insufficient evidence was submitted of biased and unequal news coverage in favor of Peña Nieto, particularly by No. 1 TV broadcaster Televisa, even though Britain's Guardian newspaper published e-mails about a secret unit at the network whose mission was to promote the PRI hopeful.

They also dismissed evidence of deliberate distortion in pre-election polls, use of illegal funding and excessive campaign spending, vote buying and coercion, improper intervention in the election by state governors and other public officials and irregularities on election day.

The TEPJF's decision showed that the tribunal was not equal to the occasion, Zambrano said.

"No one could honestly affirm that (the TEPJF's) decision comes to strengthen the rule of law or reduce the impunity that is eating away at the nation's foundations," he said.

Peña Nieto won the election 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.

But the Progressive Movement said the victory was tainted, alleging that the PRI used several front companies to purchase debit cards and then handed them out to millions of prospective voters to secure support for Peña Nieto.

Lopez Obrador's team said the PRI exceeded campaign spending limits many times over.

The governing National Action Party, or PAN, whose candidate, Josefina Vazquez Mota, finished a distant third in the presidential election, also joined with Lopez Obrador in demanding an investigation of the PRI's finances.

National Action, however, did not formally contest the election results.

The non-partisan Yo soy 132 student movement also has held a series of protests and other events to block the "imposition" of Peña Nieto as president.

After the TEPJF rejected the challenge to the election, hundreds of people protested Thursday night outside the courthouse in Mexico City, hurling security barriers and chanting slogans denouncing the PRI and Peña Nieto.

The demonstrators, who also held up signs reading "We Demand this Dirty Election Be Annulled" and "Peña Is Not President," did not spar with anti-riot police deployed nearby.

Lopez Obrador said early Friday that he did not accept the court's decision and urged supporters to engage in peaceful civil disobedience.

He called for a demonstration on Sept. 9 in the Zocalo, Mexico City's massive main square, to define the left's next steps in the defense of "citizens' individual and social rights."

PRI Chairman Pedro Joaquin Coldwell, for his part, said after the the court dismissed Progressive Movement's challenge that Lopez Obrador and his supporters should "leave behind the political confrontations inherent in every electoral contest."

Acceptance of the outcome of definitive judicial rulings is the foundation of "justice, social harmony and peace," Coldwell said Friday.

The final confirmation of the PRI's victory means that party will return to power in December after a 12-year absence.

The PRI, which governed Mexico from 1929 to 2000, lost the 2000 presidential election to the PAN and finished third in 2006.

During its 71-year reign, 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.

Lopez Obrador also came in second in the previous presidential balloting in 2006, losing by the narrowest margin in Mexican history - a mere 0.56 percent of the 41 million ballots cast - to the PAN's Felipe Calderon.

The leftist stalwart cried fraud and declared himself to be Mexico's "legitimate president," organizing a series of marches over several weeks in Mexico City's Paseo de la Reforma, one of the capital's main thoroughfares. EFE