Marco Ugarte / AP
Wearng a Guy Fawkes mask, a protester holds a banner that reads in Spanish, "No to another fraud," during a march in Mexico City on Saturday.
Updated at 7:50 a.m. ET: MEXICO CITY -- Tens of thousands of protesters, many holding hand-written placards, marched through the Mexican capital on Saturday against President-elect Enrique Pena Nieto, accusing him of buying votes and paying off TV networks for support.
The demonstrators, including students, leftists, anarchists and union members, shouted slogans criticizing Pena Nieto's Institutional Revolutionary Party, or PRI, and the electoral authority.
"Mexico without the PRI," "Mexico voted and Pena didn't win," and "If there is an imposition, there will be a revolution," chanted demonstrators, according to Spanish-language Mexican newspaper La Jornada.
The PRI previously ruled Mexico for seven decades, during which time it was accused of rigging elections and repressing protesters.
Many carried signs reading, "Pena, how much did it cost to become president?" and "Mexico, you pawned your future for 500 pesos."
"The PRI threatens many people and buys others with a couple of tacos," said Manuel Ocegueda, a 43-year-old shop worker participating in the march.
There were also demonstrations in Cancun, Puebla and Ciudad Juarez.
Accusations of vote-buying began surfacing in June, but sharpened later when people rushed to grocery stores on the outskirts of Mexico City to redeem pre-paid gift cards worth about 100 pesos ($7.50). Many said they got the cards from PRI supporters before the elections.
Officials put the size of the crowd that reached its central Zocalo plaza at 50,000.
In an interview with NBC's Spanish language network Telemundo, the apparent winner of Mexico's presidential election, Enrique Pena Nieto, spoke out about the challenges he faces. NBC's Kate Snow reports.
Pena Nieto won the July 1 election by almost 7 percentage points, according to the official count, returning the PRI to presidential power after 12 years in the wilderness. He is due to take power in December, replacing Felipe Calderon of the conservative National Action Party, or PAN.
The constitution barred Calderon from running for a second term. The PAN candidate, Josefina Vazquez Mota, finished third, with many voters dissatisfied over relentless drug violence and sluggish growth.
Leftist Andres Manuel Lopez Obrador finished in second place, but has refused to concede, accusing Pena Nieto of stealing the presidency.
Pena Nieto denies any wrongdoing and PRI officials say they could sue Lopez Obrador over the accusations.
Lopez Obrador was runner-up in 2006 as well, but by a much closer margin, after which he also said there had been fraud and led major protests that disrupted the capital city for months.
He has so far stayed away from protests this year, but said he was collecting evidence of fraud to give to officials.
On Saturday, Lopez Obrador called on the PAN to join forces with him to file a legal challenge to the election.
Mexico's electoral tribunal has until September to evaluate any complaints and officially name Pena Nieto as the next Mexican president.
In a dramatic comeback for the Institutional Revolutionary Party, Enrique Pena Nieto claimed victory in Sunday's presidential election in Mexico. NBC's Mark Potter reports.
Pena Nieto, 45, a former state governor, promises to reform Mexico's oil industry and labor laws to kick-start growth and boost security spending to reduce the country's high murder rate.
Calderon has congratulated Pena Nieto on his electoral victory, along with dozens of world leaders.
Msnbc.com's F. Brinley Bruton, Reuters and The Associated Press contributed to this report.
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