MEXICO CITY (CNN) -- More than a week after Mexico's presidential election, the candidate who authorities describe as the runner-up said a partial recount was not enough to erase his doubts about the vote.

"We cannot accept these results," Andres Manuel Lopez Obrador told reporters Monday, vowing to file a formal complaint before a tribunal on Thursday, the legal deadline for challenging the election results.

Lopez Obrador asserted that presumptive president-elect Enrique Peña Nieto and his party bought millions of votes in the election -- an accusation party officials have denied.

"I am president by the majority decision of the Mexicans," Peña Nieto told CNN last week, noting that numerous world leaders, including U.S. President Barack Obama, had called to congratulate him on his win.

An official tally of returns released after a partial recount by Mexico's Federal Election Institute last week confirmed that Peña Nieto of the Institutional Revolutionary Party, or PRI, received 38.21% of votes. Lopez Obrador, of the Democratic Revolution Party, garnered 31.59% of votes, election officials said.

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 official election results. The new president will be sworn in 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, referring to himself as "the legitimate president of Mexico." Lopez Obrador's supporters protested nationwide. In Mexico City, they organized 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.

An anti-Peña Nieto youth movement said it received 1,100 complaints of irregularities. And the group Civic Alliance said 30% of voters it surveyed reported witnessing at least one type of irregularity.

Mexican election regulators said they are investigating the allegations related to the gift cards.

PRI spokesman Eduardo Sanchez has called the claims "a farce," and accused political opponents of staging the videos and photographs purported to show vote-buying.

CNNMexico.com's Tania L. Montalvo contributed to this report.