"Rivers of illicitly obtained money were used to buy millions of votes," Lopez Obrador told a news conference Friday. He also claimed that the recount of ballots at over half of polling places had not been carried out as thoroughly as promised.
Josefina Vazquez Mota of the conservative National Action Party got 25.41 percent of votes cast, and the small New Alliance Party got 2.29 percent, barely passing the 2 percent barrier needed to preserve the party's place on future ballots.
Almost 2.5 percent of ballots where voided; while some voters void their ballots as a protest, some simply make mistakes in marking them.
The final vote count must be certified in September by the Federal Electoral Tribunal. The tribunal has declined to overturn previously contested elections, including a 2006 presidential vote that was far closer than Sunday's.
Accusations of vote-buying began surfacing in June, but sharpened early this week as thousands of people rushed to grocery stores on the outskirts of Mexico City to redeem prepaid gift cards worth about 100 pesos ($7.50). Many said they got the cards from PRI supporters before Sunday's elections.
Lopez Obrador said millions of voters had received prepaid cards, cash, groceries, construction materials, or appliances. Lopez Obrador would not rule out street demonstrations, like the one he led in 2006 to protest alleged fraud in the presidential elections of that year, which he narrowly lost to President Felipe Calderon.
But he said Thursday that his challenge of the results would be channeled through legal venues. "We have acted and we will continue to act in a responsible way, adhering to the legal procedure," Lopez Obrador said.
Leonardo Valdes, the president of the Federal Electoral Institute, said he doesn't see any grounds for overturning the results. "I do not see any justification for rejecting the entirety of the election results," Valdes said.
However, he said the institute, Mexico's chief electoral watchdog agency, had begun an investigation into the gift cards, and had requested that the PRI and the grocery store chain that issued the cards turn over information.