Thousands of people took part in a march in Mexico City to protest against Enrique Peña Nieto, of the Institutional Revolutionary Party, or PRI, who was declared the winner of last week's presidential election.
The march started Saturday at the Angel of Independence monument amid shouts of "fraud, fraud, fraud" and "Andres, hold on, the people are rising up," a reference leftist presidential candidate Andres Manuel Lopez Obrador.
Peña Nieto, according to the final tally, won the July 1 presidential election with 38.21 percent of the vote.
The results, however, have been challenged amid allegations of massive vote-buying by the PRI.
Saturday's march, which was organized via social-networking sites and dubbed the "March against Imposition," shut down traffic on the busy Paseo de la Reforma.
The "Yo Soy 132," which was created to oppose Peña Nieto's candidacy, said it did not organize the massive march.
The Federal Electoral Institute, or IFE, released its final election results on Friday, confirming that Peña Nieto won the election with 19.2 million votes, or more than 3 million votes more than Lopez Obrador, who got 31.59 percent of the vote.
Governing National Action Party, or PAN, candidate Josefina Vazquez Mota ended up in third place, winning 25.41 percent of the vote.
The figures must still be certified by Mexico's top elections court, which will have to rule on the challenges.
Lopez Obrador, the candidate of a leftist coalition led by the Party of the Democratic Revolution, or PRD, said he planned to formally challenge the outcome, based on credible allegations of vote-buying by the PRI, which became notorious for electoral chicanery during its 1929-2000 hammerlock on the presidency.
The former Mexico City mayor lost the 2006 presidential election to Felipe Calderon, of the PAN, by 0.56 percent and has never recognized the results, claiming victory in that contest.
"We are following the trail of the money that was used to buy the votes, following the trail of the money and the illicit financing that has to do with the Monex bank and the Soriana company," Lopez Obrador said in a press conference on Saturday.
PRD chairman Jesus Zambrano said at the same press conference that he planned to ask PAN chairman Gustavo Madero to exchange information about the election's financing, especially about the allegations surrounding Banco Monex.
"We have agreed to expressly and officially ask Gustavo Madero for all the information that we can exchange so, if they decide, we can go together and ask the National Banking and Securities Commission, an agency of the Finance (Secretariat), to investigate all the accounts that the money moved through," Zambrano said.
"Hopefully, the PAN will be willing to hand over the information to election authorities," the PRD chairman said.
The PAN alleged before the election that the PRI was giving its political operatives debit cards issued by Banco Monex to buy votes, drawing on an account containing 700 million pesos ($50 million).
The election councils that will distribute seats in Congress, meanwhile, began the process on Sunday, working off the official final tally.
The councils in the different states and the Federal District will decide the allocation of seats, with the decisions subject to challenge. EFE