Mexico's Yo soy 132 student movement presented Thursday a campaign aimed at persuading authorities to annul the July 1 presidential election and avert the "imposition" of ostensible winner Enrique Peña Nieto as the country's next head of state.

"Our general plan begins by contributing to the cleaning up of the electoral process, but it has as its goal the legitimate petition to declare the election invalid," one of the group's representatives, Sofia Silva, told a press conference here.

Yo soy 132 emerged in May, largely as a reaction to the Mexican mass media's bias in favor of Peña Nieto, candidate of the Institutional Revolutionary Party, or PRI.

The movement has already submitted to prosecutors and electoral authorities a report containing more than 1,000 instances of irregularities at polling places and said it will deliver a second dossier next week.

"If the authorities charged with validating the election don't take into account the hundreds of citizen complaints documented so far, they will be taking a step toward a grave risk of social explosion," Yo soy 132 said in a statement, while emphasizing their own commitment to strictly peaceful protest.

The group's spokespeople announced a "national mega-march against the imposition," urging Mexicans to fill town squares across the country on Sunday.

Peña Nieto won the election with 38.21 percent of the vote, while the standard-bearer of a leftist coalition, Andres Manuel 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 the TEPJF electoral court seeking to have the election overturned, pointing to reports of vote-buying by the PRI.

Earlier Thursday, the respective chairmen of the governing conservative National Action Party and the leftist PRD, Gustavo Madero and Jesus Zambrano, held a press conference in Mexico City to demand an investigation of the PRI for possible money laundering.

The party chiefs also disclosed that they asked electoral authorities to complete their probe of PRI campaign finances before the Aug. 31 deadline for the TEPJF to certify Peña Nieto as president-elect.

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 years of largely unchallenged hegemony, 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 Felipe Calderon by 0.56 percent of the vote, a result the leftist candidate refused to accept. EFE