Thousands of people rushed to stores on Tuesday to redeem pre-paid gift cards they said were given them by the party that won Mexico's presidency, inflaming accusations that the election was marred by massive vote-buying.

At least a few cardholders were angry, complaining they didn't get as much as promised, or that their cards weren't working.

Neighbours at one store in a poor area on the outskirts of Mexico City said the unusually large crowds had prevented them from doing their daily shopping.

Some of those lined up to use their gift cards said they got them for supporting the Institutional Revolutionary Party, or PRI, whose candidate Enrique Pena Nieto won Sunday's election, according to the preliminary official vote count. Some wore red T-shirts and baseball caps with Pena Nieto's name printed in white.

Maria Salazar, a 20-year-old university student, came with her 70-year-old father, Antonio Salazar, to cash three gift cards.

"They gave us the cards in the name of the PRI and Representative Hector Pedroza [a PRI congressional candidate], and they said they were counting on our vote," Maria Salazar said outside the store, as she carried plastic shopping bags packed with toilet paper, cooking oil, rice, saltine crackers and instant noodle soups.

Her father carried another two packed grocery bags and her eight-year-old nephew carried another.

"They told us they were worth 500 pesos [$37.50 US], but when we got to the check-out, they were only worth 100 rotten pesos [$7.50]," Salazar said.

Both she and her father said they had been told to turn in a photocopy of their voter ID card in order to get the gift cards.

Another woman interviewed outside the same Soriana grocery store also complained her card had only 100 pesos in credit.

"For helping them with votes and all … they gave us a card for supporting them, and all that for 100 pesos," said the woman, who gave only her first name, Josefina, for fear of reprisals. She said she got the card for supporting Pena Nieto, but complained that "100 pesos lasts you about five minutes."

Inside the store, such long lines formed at card-reading machines as people tried to find the balances on their cards. Some grew angry and shouted insults against Pena Nieto.

Regular shoppers were vexed at the long lines. "I was going to buy bread right now, but you can see, the lines are tremendous, you can't even get in," said Maria Garcia Lobato.

No comment from PRI

Pena Nieto's campaign and the PRI press office said they had no immediate comment, and the press representative of the Soriana grocery store chain did not immediately respond to phone calls. In the final days of the campaign, PRI officials denied similar allegations that the party distributed pre-paid cash cards from a local bank.

Institutional Revolutionary Party, or PRI, candidate Enrique Pena Nieto won Sunday's presidential election in Mexico, according to the preliminary official vote count. (Claudi Daut/Reuters)

On the Friday before the vote, the leftist Democratic Revolution Party — whose candidate Andres Manuel Lopez Obrador came in second — issued a statement accompanied by photos of dozens of the Soriana cards, saying they had been distributed by a PRI-affiliated union, and it filed a complaint to electoral authorities.

Under Mexican election law, giving voters gifts is not a crime unless the gift is meant to condition or influence their votes. Also, the cost of such gifts must be reported to authorities, and cannot exceed campaign spending limits. Violations of those rules are usually punished with fines, but are not usually considered ground for annulling the elections.

On Tuesday, Alfredo Figueroa, a council member of the oversight agency known as the Federal Electoral Institute, said authorities are investigating the Soriana card complaint. Members of the institute have said they were aware of attempts to engage in vote buying.

Figueroa also said that irregularities in vote tallies may eventually lead to the opening and recounting of votes from as many as 50,000 polling stations, about one-third of the 143,000 involved in Sunday's vote.

'National embarrassment'

But on Tuesday, Lopez Obrador said his team had detected irregularities in 113,855 polling places, and called for a much wider recount. "This is a scandal.… They bought millions of votes," Lopez Obrador said, referring to the PRI. "Clearly, they far exceeded campaign spending limits … this is a national embarrassment," he told a news conference.

Lopez Obrador has refused to accept the preliminary vote tallies, saying the election campaign was marred by overspending and favourable treatment for Pena Nieto by Mexico's semi-monopolized television industry.

Andres Manuel Lopez Obrador, presidential candidate for the Democratic Revolution Party, has said he will wait until Wednesday, when the official results are to be announced, before deciding what he will do. (Dario Lopez-Mills/Associated Press)

Many also questioned why pre-election polls showed Pena Nieto with a double-digit lead, roughly twice as large as the margin he really won by. With 99 per cent of the vote tallied in the preliminary count, Lopez Obrador trailed by just six percentage points.

The narrower-than-expected margin is fuelling suspicion among Lopez Obrador's followers about the fairness of the vote, and he refused Monday night to concede defeat, just as he did when he lost a razor-thin race in the 2006 presidential race and set off months of political unrest. Although this time, he has not called his followers into the streets to protest.

Lopez Obrador argued from the start of the campaign that pollsters were manipulating surveys to to promote the idea that the PRI candidate was far out in front.

Pollsters deny that, saying they believe some voters switched to Lopez Obrador in the final week, a period when publication of new polls is banned by law.

Lopez Obrador said he would not accept the preliminary election results reported by the Federal Elections Institute and would wait until Wednesday, when the official results are to be announced, before deciding what he will do.

"We will not accept a fraudulent result," Lopez Obrador said.