The Mexican left says the July 1 presidential election must be overturned and an interim president appointed to organize another vote.
"If the election is not invalidated, Mexico will be governed by a band of criminals, the most corrupt and terrible in the country," the leftist who finished second in the presidential contest, Andres Manuel Lopez Obrador, said at a press conference in the capital.
Enrique Peña Nieto, the candidate of the Institutional Revolutionary Party, or PRI, won with 38.21 percent of the vote, while Lopez Obrador took second place with 31.59 percent, according to the final official tally.
But the leftist hopeful filed a motion last week with Mexico's TEPJF electoral court seeking to have the election overturned, contending that the PRI effectively bought 5 million votes.
The TEPJF has until the end of August to either officially certify Peña Nieto as president-elect or annul the election.
"We should not be surprised that the election is canceled, that it's invalidated," Lopez Obrador said Thursday. "The constitution establishes the procedure for these cases."
The lower house of Congress chooses an interim "honorable president to convene new elections within a period of 15 to 18 months, at the latest," he said, rejecting the notion that such a process would lead to "instability."
"I assure you that it will be worse for us as a nation if there is impunity, if it's decided there will be a president ... who comes to office by buying the presidency of the republic," Lopez Obrador said.
Though Mexico's conservative governing National Action Party has joined the leftist coalition in demanding an investigation of the PRI's finances, Lopez Obrador says the administration of outgoing President Felipe Calderon is concealing evidence of wrongdoing by the Peña Nieto campaign.
The PRI, which governed Mexico from 1929 to 2000, lost the 2000 presidential election to National Action and finished third in 2006.
During its 71-year reign, the PRI relied mainly on patronage and control of organized labor and the mass media, though it was not above resorting to outright vote-rigging and even violence.
Despite their ideological differences, National Action and the PRD have a history of working together at the state and local levels to battle the PRI.
That relationship was put under strain when Lopez Obrador lost the 2006 contest to National Action's Calderon by 0.56 percent of the vote, a result the leftist candidate refused to accept.
It is not just the organized left that is challenging the outcome of the July 1 ballot, as the non-partisan Yo soy 132 student movement has announced a series of protests and other events to block the "imposition" of Peña Nieto as president.
Thursday night, thousands of Yo soy 132 members and supporters surrounded the Mexico City headquarters of Televisa, the country's dominant television broadcaster.
"The symbolic and peaceful seizure" began around 9:30 p.m. after a march that was delayed by a torrential downpour.
Besides chanting slogans against Peña Nieto and the PRI, protesters held up signs saying: "Televisa lie factory" and "We want schools, not soap operas."
Yo soy 132 emerged in May, largely as a reaction to the Mexican mass media's bias in favor of Peña Nieto and the PRI. EFE