Mexico's political left said several state governors, mostly members of the Institutional Revolutionary Party, or PRI, were behind a massive vote-buying scheme in last Sunday's general election.

"There's no logic in terms of the participation that was seen in certain regions of the country, mainly where the PRI governs," the presidential candidate of a leftist coalition headed by the Party of the Democratic Revolution, or PRD, Andres Manuel Lopez Obrador, said in a press conference Friday.

Speaking at his campaign headquarters a day after the Mexican left said it will formally challenge the election results, Lopez Obrador reiterated that he will provide evidence that "those who don't want change in the country bought the election."

"We're talking about very poor areas, both rural and urban," he said, adding that vote buying was "out in the open, immoral, shameless."

Electoral authorities said Friday that the PRI's Enrique Peña Nieto won the July 1 presidential election with 38.21 percent of the vote, while Lopez Obrador garnered 31.59 percent.

The victory for the PRI, which governed Mexico uninterruptedly from 1929 to 2000, returns that party to the presidency after a 12-year absence.

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.

Although many Mexicans remain suspicious of the party due to its corrupt past, the PRI was able to regain power in large part due to spiraling drug-related violence that has left more than 50,000 dead during the presidency of the National Action Party's Felipe Calderon, who took office in late 2006.

Lopez Obrador, for his part, finished runner-up for the second consecutive presidential election and maintains that both contests were marred by fraud.

After losing to Calderon by a razor-thin margin in 2006, Lopez Obrador refused to accept the result and had himself proclaimed as "legitimate president" before tens of thousands of supporters in Mexico City.

On Friday, the veteran politician said "anti-democracy" forces cannot impose themselves but stressed that he and his supporters will act "responsibly" and did not say if he would take to the streets to protest the results.

"We're going to keep acting peacefully. We don't want confrontation. We're right and we're demonstrating that and it's going to be fully demonstrated," he said Friday.

He also did not indicate whether he will join the protests of the nonpartisan "Yo Soy 132" student movement, which arose in May to demand that Mexico's leading television network Televisa stop slanting campaign coverage in favor of the PRI.

That student movement has compiled more than 1,000 complaints of irregularities in the election, the vast majority allegations against the PRI. The most common reported irregularities include the buying of votes and voter credentials and violations of a ban on campaigning in the days leading up to the election.

Lopez Obrador said Friday the student movement is independent and composed of young people who "have acted very responsibly and creatively."

He added that he and his team will gather evidence this weekend and present a detailed report to Mexican society on how the presidential election was "bought."

The goal, he said, is to "clean up the election, defend democracy, defend citizens' votes and not accept any electoral fraud."

Lopez Obrador urged electoral authorities not to make public statements indicating the election results are set in stone, adding that "they must act as judges and uphold the constitution."

"We've got to wait and see. They made it easy for them to buy votes, easy for them to use the media to impose Peña Nieto, but it's not enough for them to say 'we made a mistake,'" he said.

For its part, the chairman of the conservative National Action Party, or PAN, which has held the presidency since 2000, said the PRI won the election with "wads of cash and trickery" but insisted his party will respect the election results.

Gustavo Madero said in a press conference in Mexico City Friday that "fraud no longer takes place the day of the election, but ... in the lead-up, as far back as six years or more, to pave the way for this result."

The PAN's standard-bearer, Josefina Vazquez Mota, finished a distant third with 25.41 percent of the vote.

Madero said his party will challenge irregularities such as the breaching of campaign spending limits, direct and indirect vote buying and campaigning past the allotted time period in certain areas of the country but not lodge an overall challenge to the voting results as the left has vowed.

He also denounced the "illegal participation" of state governments in the election and the political manipulation of voter preference polls in the media, which "did not contribute to informing citizens in a truthful manner."

For his part, Peña Nieto was quoted as saying in a PRI statement Friday that the allegations of electoral chicanery "must be proven in the (Federal) Electoral Tribunal," which still must ratify the results.

The presumed president-elect said last Sunday's election was notable for the "large participation of Mexicans, with more than 50 million voters heading to the polls" and said that lent "certainty" to the process.

"Isolated irregularities" obviously could have occurred, but the election was the "most closely watched in Mexico's history, with 3 million citizens and party representatives" maintaining vigilance over 143,000 polling stations, he added.

"I recognize the right of other parties and candidates to have recourse to the law to resolve any type of doubts that may arise, but the transparency of the electoral process and Mexican society's support for my project are undeniable," Peña Nieto said.

By law, political parties have from July 9 to July 12 to formally challenge the election results. EFE