(CNN) -- An official tally of Mexico's presidential election returns -- including individual recounts for more than half the ballots -- confirmed Enrique Peña Nieto as the winner of the election.
But until the country's electoral tribunal ratifies the results -- and challenges are virtually assured -- Peña Nieto remains the presumptive president-elect.
According to the website of the Federal Electoral Institute, with 100% of the totals counted, Peña Nieto of the Institutional Revolutionary Party, or PRI, had 38.21% of vote, while leftist Andres Manuel Lopez Obrador finished with 31.59% and conservative candidate Josefina Vazquez Mota had 25.41%.
The official tally included individual ballot recounts in 54.5% of the country's electoral districts.
Lopez Obrador has vowed to challenge the results, accusing the PRI of vote-buying, and said that he would take his complaints through the legal system. He did not immediately comment following the announcement of the official count.
For his part, Peña Nieto told CNN that "I am president by the majority decision of the Mexicans."
If Lopez Obrador, who was the candidate for a leftist coalition, continues his challenge, the process could stretch out for months.
The federal electoral tribunal, known as TRIFE, will begin on Monday to accept complaints of voting irregularities.
The tribunal will have until September 6 to complete its investigation and ratify -- or reverse -- the official election results. The new president will be sworn in on December 1.
In the 2006 presidential vote, election authorities said Lopez Obrador narrowly lost to Felipe Calderon. The former Mexico City mayor claimed election fraud and never conceded, referring to himself as "the legitimate president of Mexico."
Lopez Obrador's supporters protested nationwide. In Mexico City, they staged sit-ins and blockades.
Officials have called this election the most transparent in Mexico's history. It was the first election in which scanned copies of district-by-district election returns were posted on the Internet.
But accusations have arisen of electoral manipulation by the PRI.
The party, which was in power for 71 years before being voted out in 2000, was known for being corrupt and authoritarian.
Its apparent victory last weekend would mark a triumphant return after 12 years of rule by the right-wing National Action Party, known by the acronym PAN.
Opponents of the PRI said they have video and photo evidence of the party buying votes through thousands of cards that could be redeemed for products at a chain of supermarkets.
An anti-Peña Nieto youth movement said it received 1,100 complaints of irregularities. And the group Civic Alliance found in a survey it conducted that 30% of voters reported witnessing at least one type of irregularity.
Mexican election regulators said they are investigating the allegations related to the gift cards.