The runner-up in Mexico's presidential race has filed a legal complaint before the country's electoral court seeking to nullify results that handed the centrist Institutional Relovolutionary Party (PRI) victory.
Andres Manuel Lopez Obrador, who finished second with 3.3 million votes behind the PRI's Enrique Pena Nieto, says the winner broke campaign finance laws and bought millions of votes.
The former mayor of Mexico City, who lost the 2006 presidential race by a narrower margin, says major media outlets skewed coverage to promote the telegenic Pena Nieto who finished first with 38 per cent of the votes.
Lopez Obrador, who was backed by the Party of the Democratic Revolution (PRD) and a coalition of smaller left-wing parties, said he had proof Pena Nieto colluded with local governors to buy around five million votes.
He said his opponents doled out pre-paid gift cards, cash, foodstuffs, building materials and fertilizer to lure poor voters to the ballot boxes.
Lopez Obrador's lawyers filed the complaint with election officials on Thursday evening, delivering dozens of boxes filled with documents, videos, photos as well as home appliances, clothes and kitchen utensils allegedly given away by the PRI campaign.
The court has until early September to deal with any challenges and determine whether to validate the July 1 election. Ruling party candidate Josefina Vasquez Mota conceded defeat after trailing with 25 per cent.
Lopez Obrador said that he would reveal next week what he called a "national plan for the defence of democracy and the dignity of Mexico.
"He did not give any hints on what the plan would entail, but said he and his supporters would act peacefully.
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"I call on all Mexicans not to allow the shameless violation of the constitution," Lopez Obrador said. "We will always act peacefully; we won't give them any excuses to accuse us of being violent."
"The massive vote buying operation was carried out before and on the day of the election," Lopez Obrador told a news conference.
In 2006, Lopez Obrador also refused to accept his loss to President Felipe Calderon by less than 1 percentage point. Financial 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.
Pena Nieto, 45, is set to return the PRI to power after 12 years in opposition. The party ruled Mexico for seven decades straight and often turned to authoritarian tactics to stifle political rivals and rig elections.
Lopez Obrador said the party resorted to its old tricks after polls showed he was gaining ground late in the campaign and a student-led movement began to rally against Pena Nieto's candidacy.