By Lizbeth Diaz

MEXICO CITY, July 9 (Reuters) - The runner-up in Mexico's presidential election rejected the final results of the contest on Monday and said he had evidence that about 5 million votes had been bought by opponents.

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 in 2006. "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 concentrated heavy fire on the PRI in the past week with allegations of vote buying.

Lopez Obrador said he would wait until Thursday to decide his next step but said he is considering asking that the election be annulled.

After the 2006 election Lopez Obrador staged weeks of protests that blockaded parts of Mexico City but has not called for demonstrations to protest this election. Still, on Saturday, thousands marched in the capital to voice their discontent with Pena Nieto and the PRI over the election.

Outgoing President Felipe Calderon of the conservative National Action Party said on Monday vote buying unacceptable and 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 retail stores to buy support from voters.

"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.

Mexico's election tribunal will have until September to rule on any wrongdoing.

Lopez Obrador said Saturday he is gathering evidence for a legal challenge and urged the PAN to join him. The PAN is still deciding on how to respond.

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. It was the dominant force in Mexican politics during the past century and it held 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)