(CNN) -- The runner-up in Mexico's presidential election announced Thursday that he was filing a legal challenge to invalidate the vote.
Andres Manuel Lopez Obrador of the Democratic Revolution Party has said that presumptive President-elect Enrique Pena Nieto and his party bought millions of votes in the election, an accusation that party officials have denied.
Lopez Obrador has said a partial recount was not enough to erase his doubts about the vote.
"To proceed in another way would be to give up our fundamental rights," Lopez Obrador told reporters at a news conference.
A tally of returns released by Mexico's Federal Election Institute last week confirmed that Pena Nieto of the Institutional Revolutionary Party, or PRI, received 38.21% of the votes. Lopez Obrador garnered 31.59% of the votes, election officials said.
The tally has been completed, but the results remain unofficial until the country's electoral tribunal ratifies them.
The tribunal will have until September 6 to complete its investigation and ratify -- or reverse -- the election results. The new president will be sworn in on December 1.
Lopez Obrador has criticized the election and refused to concede repeatedly over the past week, echoing comments he made in 2006 when election authorities said the leftist candidate narrowly lost the presidential vote to Felipe Calderon.
After that election, the former Mexico City mayor claimed election fraud and never conceded.
Back then, Lopez Obrador called himself "the legitimate president of Mexico," and his supporters protested nationwide. In Mexico City, they staged sit-ins and blockades.
Officials have called this year's 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.
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.
The PRI has called the claims a farce and accused political opponents of staging the videos and photographs purported to show vote-buying.
CNN's Samira Said and Nick Parker contributed to this report.