Mexican electoral officials are recounting votes from more than half of the 143,000 polling stations in Sunday's presidential and congressional elections, in response to claims of widespread fraud and other irregularities.

A spokesman for Mexico's Federal Electoral Institute, the IFE, said 78,000 ballot boxes will be reopened this week. Edmundo Jacobo announced the recount Wednesday, just hours after initial tallies showed candidate Enrique Pena Nieto, of the Institutional Revolutionary Party (PRI), winning the presidency by nearly 7 percentage points over his nearest rival.

However, runner-up Andres Manuel Lopez Obrador refused on Tuesday to concede, after accusing the PRI party of vote buying and coercion. Obrador, a leftist, also accused Mexican news media of extending favorable coverage to the PRI, which ruled the country for seven decades until 2000.

Tuesday's allegations of widespread vote buying were fueled by scenes of thousands of people rushing to grocery stores to redeem pre-paid gift cards they said the PRI had given them ahead of Sunday's vote. A shopper identified as Josefina explains.

For helping them (the PRI) with votes and all, they gave us a (gift) card for supporting them, and all that for 100 pesos, and 100 pesos are gone in five minutes.”

Local Soriana resident Elisa voiced displeasure with the alleged giveaway.

“I think this is an abuse and undignified. It's dirty that a person running for the presidency takes advantage of hunger, ignorance and poverty and gives away money in cards that people are now using. He gives away money to buy his votes.”

The PRI has denied any wrongdoing. But Obrador said his workers had detected irregularities at more than 100,000 polling stations.

Mexico's Interior Secretary Alejandro Poire, offered a different analysis of Sunday's polls.

“”We would qualify this election as taking place in peace, with security, with transparency and within the law.”

In 2006, Lopez Obrador demanded a recount after losing the presidency to Felipe Calderone by slightly more than half a percentage point. His requests were refused, triggering protests by the candidate's supporters that choked Mexico City for weeks.