By Lizbeth Diaz
MEXICO CITY (Reuters) - The runner-up in Mexico's presidential election rejected on Monday the final results of the contest and said he had evidence that about 5 million votes had been bought by the winner's political party.
Sunday's official tally said leftist Andres Manuel Lopez Obrador, who led six weeks of protests when he lost the 2006 presidential election, finished second with 31.59 percent of the vote.
That left him about 3.3 million votes behind winner Enrique Pena Nieto of the Institutional Revolutionary Party (PRI) with 38.21 percent.
International election observers said the July 1 vote had been clean. But prior to polling day Lopez Obrador had accused Pena Nieto and the PRI of vote buying, raising the prospect of a legal dispute over the outcome that could last for weeks.
"We cannot accept those results," said Lopez Obrador, who also said he was a victim of voter fraud six years ago. "We have evidence to support this and when the time is right we are going to prove that around 5 million votes were bought."
The fiery orator, who ran as a candidate of a coalition of leftist parties led by the Party of the Democratic Revolution (PRD), did not specifically point the finger at Pena Nieto on Monday.
But he has focused on the PRI in the past week with allegations the party handed out gifts and shopping cards to lure poor voters to the polls.
Lopez Obrador said he would wait until Thursday to decide his next step. He is considering asking that the election be annulled.
"I think it is in Lopez Obrador's interest for it to drag on," said Christopher Sabatini from the Washington-based Council of the Americas, adding that it would be nearly impossible for the PRD to prove there was massive vote buying.
Lopez Obrador, who staged weeks of protests after the 2006 election that snarled parts of Mexico City, has not called for demonstrations to protest this election.
Still, on Saturday, thousands marched in the capital to express their discontent with Pena Nieto and the PRI over the election.
'VICE' OF MEXICAN DEMOCRACY
Outgoing President Felipe Calderon of the conservative National Action Party (PAN) said on Monday that the accusations must be looked into.
"The essence of democracy isn't only counting votes but also that the campaigns play out in fair conditions," he said in a radio interview, adding that vote buying was a "vice" of Mexican democracy that needed to be corrected immediately.
One of Lopez Obrador's main complaints is that the PRI used gift cards for retailer Soriana to buy support from voters. Soriana fiercely denies it was involved in any way.
"The business of the cards and the accounts and all, I'm not saying it's going to be enough to change the results of an election with such a difference (in the number of votes), I don't know, but it needs to be resolved," Calderon said.
The PRD's national chairman said the PRI spent nearly six times more than the 336 million pesos ($25 million) legally permitted in the campaign.
Mexico's election tribunal will have until September to rule on any wrongdoing.
Lopez Obrador said on Saturday he was gathering evidence for a legal challenge and urged the PAN to join him. But Cecilia Romero, the PAN's general secretary said the party will not seek to annul the election.
Pena Nieto denies wrongdoing and PRI officials have threatened to sue Lopez Obrador over his accusations.
If the election result stands, Pena Nieto's win will return the PRI to the presidency after 12 years in opposition. The party was the dominant force in Mexican politics during the past century, holding the presidency from 1929 until 2000. The PRI's long rule was marred by frequent accusations of corruption, vote-rigging and other abuses.
(Additional reporting by David Alire Garcia and Dave Graham; Editing by Bill Trott and Xavier Briand)