The announcement came after Lopez Obrador, who came in second, claimed "fraudulent" results
Mexico's national election authority announced a recount of ballots cast at more than half of the polling stations in the country's presidential election in which Enrique Pena Nieto claimed victory.
Edmundo Jacobo, a top official with the Federal Electoral Institute (IFE), made the announcement one day after leftist candidate Andres Manuel Lopez Obrador, who came in second in the vote, demanded a full vote recount, claiming "fraudulent" results.
Jacobo also said they are expecting final official results for the presidential election completed as early as Thursday.
Lopez Obrador, who heads a leftist coalition led by the Party of the Democratic Revolution (PRD), accused Pena Nieto's Institutional Revolutionary Party (PRI) of orchestrated a massive scheme to influence the vote that included distributing 1.8 million gift cards.
The first official results from Sunday's vote showed Pena Nieto with 38 percent of the vote against 31 percent for Lopez Obrador.
When asked about the vote challenge, Pena Nieto told the US network CBS Wednesday that "the main threat to democracy is disenchantment from the lack of results."
Voters in the presidential and legislative elections Sunday allegedly showed PRI officials cell phone pictures of their paper ballot to "prove" they voted for the PRI, and received gift cards in return.
"There is no president-elect," said Lopez Obrador's campaign coordinator Ricardo Monreal, said Wednesday. "There is no candidate that has been elected, and whomever tries to use that title is an impostor."
Observers fear Lopez Obradors refusal to concede could trigger a repeat of the 2006 presidential election, when he lost by less than one percent, claimed fraud and organized mass protests that paralyzed Mexico City for more than a month.
"Just look at what happened in the Soriana stores ... with the panic purchases from millions of cards of people who went to cash in" on their vote, Monreal said on a local radio show.
Soriana, a major Mexican store chain, printed a full-page ad on Wednesday after their shops were swamped by people rushing in with gift cards they apparently feared might be canceled soon by the authorities.
It is "absolutely false" that cards with the logo of the Confederation of Mexican Workers, a major pro-PRI union, contained cash, "because those cards only offer discounts and loyalty points," Soriana said.
Two Soriana stores in working class Mexico City neighborhoods were forced to close due to the crush of customers with gift cards, Civil Defense officials said.
Pena Nieto's campaign coordinator Luis Videgaray insisted that the PRI won a clean victory. "If the votes are counted again, we'll still come out winners," he said.
PRI spokesman Eduardo Sanchez flatly rejected the gift card charges.
"We have not handed out any cards. Neither the Enrique Pena Nieto campaign nor the PRI have any relationship with those cards," he said.
The PRI was synonymous with the Mexican state as it governed for seven decades until 2000 using a mixture of pervasive patronage, repression, rigged elections and bribery.
Before the vote the PRD filed an official complaint over alleged PRI gift cards to be used at PEMEX gasoline stations. Separately, the PAN filed a complaint over alleged PRI bank cash cards. The PRI in turn filed complaints against both its rivals alleging gifts to voters.
The percentage difference in Sunday's vote amounts to some three million votes, election officials say, a wider margin than in the last presidential election but closer than pre-vote surveys had indicated.
"It makes little sense to carry out a recount," said Jose Antonio Crespo with the CIDE graduate school. "That was the problem in 2006."
The vote buying charges are more serious, but "it will be hard to prove" that enough votes were bought to influence the outcome.
The IFE has until September 6 to resolve balloting complaints and formally announce the winner.
Leaders around the world, including US President Barack Obama, have already congratulated Pena Nieto on his apparent victory. So did President Felipe Calderon.
"Mexico has a new president," former president Vicente Fox -- the PAN member whose 2000 election ended seven decades of PRI rule -- wrote in his blog Wednesday. "Today the democrats are counted and close ranks behind the president-elect without hesitation."
Fox unleashed a firestorm of controversy by supporting Pena Nieto and not his party's candidate in the election. PAN members are now considering kicking him out of the party.