Mexico's highest electoral authority declared Friday that Enrique Pena Nieto was the legitimate winner of the July 1 presidential election, formally opening the transition to a new government despite continuing claims of fraud by the left's second-place finisher.

The Federal Electoral Tribunal said leftist candidate Andres Manuel Lopez Obrador failed to prove claims that vote-buying had affected the results of the vote that returns the former autocratic ruling party to Mexico's highest office after a 12-year absence.

Pena Nieto, 46, insists his Institutional Revolutionary Party, the PRI, has changed. In the final decades of the 20th century, its rule was marked corruption, vote fraud and periodic economic crises.

"Mexico will have a modern, responsible presidency, open to criticism, willing to listen and take into account all Mexicans," Pena Nieto said at a ceremony in which the tribunal gave him the document certifying him as president-elect.

Outgoing President Felipe Calderon called Pena Nieto to congratulate him and wish him the best for his administration that will begin when he takes office Dec. 1. Calderon "offered his support so that that administration will be successful," the president's office said in a statement.

'The elections were not clean, free and genuine. As a result, I will not recognize an illegitimate power that's emerged as a result of vote-buying.'—Leftist candidate Andres Manuel Lopez Obrador

Lopez Obrador told reporters Friday morning that he refused to recognize the election results and was calling for a peaceful protest that he described as "civil disobedience" on Sept. 9 in the Zocalo, the historic plaza in the heart of downtown Mexico City. He launched street demonstrations that paralyzed central Mexico City after he lost the 2006 vote, but widespread protests appear far less likely this time.

Lopez Obrador said the electoral tribunal made an illegitimate ruling Thursday evening when it rejected the leftist's allegations of vote-buying and other campaign violations by the PRI. The seven electoral magistrates are nominated by Mexico's Supreme Court and confirmed by Congress and are widely seen as credible and non-partisan, although Lopez Obrador has alleged that several members were based in favour of the PRI.

"I am telling the people of Mexico that I cannot accept the judgment of the electoral tribunal that declared the presidential election valid," Lopez Obrador said at a news conference. "The elections were not clean, free and genuine. As a result, I will not recognize an illegitimate power that's emerged as a result of vote-buying and other grave violations of the constitution and the law."

Outrage at PRI win fading

Lopez Obrador, a popular former mayor of Mexico City, was able to call hundreds of thousands into the streets for campaign rallies, and he retains a large and fervent base of support in the capital. But Pena Nieto's margin of more than three million votes was far wider than the few hundred thousand votes that cost Lopez Obrador the last presidential vote, and many opponents' outrage at Pena Nieto's win appears to have largely faded since the July 1 vote.

Lopez Obrador said he wants the protest to respect the law, and he did not indicate that there would be a repeat of the blockades he launched in 2006.

By Friday afternoon, there were a few scattered protests around the capital by Lopez Obrador sympathizers, including a brief blockage of highway toll booths by a group of students, but little evidence of widespread mobilization.

Mexico's Roman Catholic Council of Bishops issued a statement calling on politicians to leave post-electoral disputes behind and unite to fight poverty and violence.

"It's time for peace, harmony and agreement," according to the bishops' statement. "We want our representatives to demonstrate that they can work together."

Confirmation of the PRI's victory returns the party to Mexico's highest office, which it held without interruption from 1929 to 2000. In past decades, the party engaged in widespread coercion of its opponents, monopolizing virtually every institution in the country. The party says it has reformed and handed control to a new generation of democratically minded young technocrats with a vision of modernizing Mexico.

Pena Nieto has promised to focus on fiscal reform, infrastructure improvements and a new emphasis on preventing violence from affecting ordinary Mexicans as a result of the country's six-year militarized offensive against drug cartels.

For much of his campaign, Lopez Obrador tried to move away from the angry, combative image that many Mexicans held of him after his supporters blockaded much of downtown Mexico City for weeks after his narrow loss in 2006. He adopted the slogan "Abrazos, No Balazos," or "Hugs, Not Bullets," put forth a warmer persona, a more business-friendly platform and an anti-crime program that relies largely on increased jobs and education programs.

An unexpectedly close vote

Lopez Obrador ended up with 31 per cent of the vote, to Pena Nieto's 38 per cent, after months of polls gave the PRI candidate a lead as wide as 20 percentage points.

The unexpected closeness of the race helped fuel Lopez Obrador's lengthy post-election fight to invalidate the results, with him and his backers accusing the PRI camp of a range of violations including the vote-buying with both gift cards and, in rural areas, farm animals, and participating in an international campaign finance money-laundering scheme.

The accusations centred on hundreds and possibly thousands of pre-paid gift cards that shoppers at a Mexican grocery store chain said they were given by Pena Nieto's party before the election. Lopez Obrador's Democratic Revolution Party showed reporters thousands of such gift cards, but never publicly demonstrated convincing evidence that millions of votes had actually been swayed by corrupt practices.

The PRI said in a statement Friday that the ruling "has ended the contentious and combative phase of the federal electoral process and has fully demonstrated the legitimacy of Enrique Pena Nieto's victory at the ballot box."

The electoral justices said some of the evidence submitted was hearsay, or unclear. For example, they said the evidence included gifts allegedly given out by the PRI, without proof that was where they came from or that the gifts had been given to influence votes.

"The evidence absolutely didn't support annulling an election with a difference of three million votes between first and second place," said Jose Antonio Crespo, an analyst at the Center for Economic Studies.

Ivan Garcia Garate, a law professor at Iberoamerican University in Mexico City, said, however, that the electoral tribunal had adopted a very narrow view of its mandate and failed to conduct its own investigation of the charges, relying entirely on evidence presented by the left, and then declaring it insufficient.