Lawyers for the runner-up in Mexico's presidential election have said that he will ask the electoral tribunal to void the results, arguing that the winner violated campaign finance laws to buy votes.
"In our opinion, with all of the violations that we have seen, it is impossible to describe these elections as free and fair," said Ricardo Mejia, the spokesman for Lopez Obrador's legal team preparing the challenge, on Thursday.
Left-wing candidate Andres Manuel Lopez Obrador came in 3.3 million votes behind Enrique Pena Nieto from the centrist Institutional Revolutionary Party (PRI), according to the official count from the July 1 vote.
But the former mayor of Mexico City, who also lost the 2006 presidential race by a narrower margin, says the campaign was rigged, with major media outlets skewing coverage to promote the telegenic Pena Nieto at the expense of the other parties.
Obrador's legal team will claim before the electoral tribunal, known as the TRIFE, that Pena Nieto violated the constitutional protections for free elections, Mejia said.
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The TRIFE has until September 6 to consider all claims and officially declare a President-elect.
In 2006, Lopez Obrador also refused to accept his loss to President Felipe Calderon by less than one per centage point.
Markets were rattled when his supporters staged weeks of disruptive protests, occupying the capital's main boulevard.
This time markets have largely shrugged off the possibility of a drawn-out conflict and Pena Nieto is already naming advisers to work on his government's transition.
"The markets have already priced in Enrique Pena Nieto winning. We don't think this challenge will change anything," said Virgilio Velazquez, at brokerage Intercam in Mexico City.
Pena Nieto, 45, will bring the PRI back to power after 12 years in opposition, reviving memories of the way it ruled Mexico for seven decades straight, often with authoritarian tactics to stifle political rivals and rig elections.
Lopez Obrador says the party has not changed its ways, and spent millions to buy building materials, food and pre-paid shopping cards to lure poor voters to the ballot boxes.
Backed by the Party of the Democratic Revolution (PRD) and a coalition of smaller left-wing parties, Lopez Obrador also claims the PRI pressured local officials to line up votes for the party in the many states where it holds power.
Mejia said the lawyers will present proof that Pena Nieto spent 4 billion pesos ($296m) during the campaign, well beyond the $24.5m allowed by Mexican law.
The PRI denies the accusations and has filed a legal complaint against Lopez Obrador for making false statements.
Pena Nieto, the former governor of the State of Mexico, won by about 6.6 per centage points and the PRI failed to clinch a majority in either the Senate or the lower house of Congress, although he was marked as the clear frontrunner all along.
Lopez Obrador said the polls are a clear sign of the media propping up Pena Nieto's campaign. At least one of the major pollsters has since admitted the readings were inaccurate.
Media reports pointed to the country's largest broadcaster Televisa receiving payments to promote Pena Nieto when he was governor and openly opposing Lopez Obrador's first presidential bid. Both the PRI and Televisa deny the claims.
Calderon's National Action Party (PAN), whose candidate Josefina Vazquez Mota finished third, voiced concerns about vote buying and media manipulation but said it would not join forces with Lopez Obrador in his push to annul the results.