MEXICO CITY — Thousands of people rushed to stores Tuesday to redeem prepaid gift cards they said were given to them previously by the party that won Mexico's presidency, inflaming accusations that the weekend election was marred by widespread vote-buying.

At least a few cardholders were angry, complaining that they didn't get as much as promised or that their cards weren't working. Neighbors at one store in a poor neighborhood on the outskirts of Mexico City said the unusually large crowds prevented them from doing their daily shopping.

Under Mexican election law, giving voters gifts is not a crime unless the gift is conditioned on a certain vote or meant to influence a vote. However, the cost of such gifts must be reported, and cannot exceed campaign-spending limits. Violations are usually punished with fines but generally aren't considered grounds for annulling an election.

Some of the people lined up to use gift cards said they got them for supporting the Institutional Revolutionary Party, or PRI, whose Enrique Pena Nieto won Sunday's presidential 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 cards.

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

Her father carried two more packed grocery bags, and her 8-year-old nephew carried another.

"They told us they were worth 500 pesos ($37.50), but when we got to the checkout, 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 also complained that her card had only 100 pesos ($7.50) 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. Before the election, the PRI accused the conservative National Action Party, or PAN, whose candidate ran third in the presidential election, of passing out groceries during the campaign, and claimed the leftist Democratic Revolution Party, or PRD, had gotten illegal campaign financing. None of those allegations have been proven.

On the Friday before the vote, the PRD issued a statement accompanied by photos of dozens of the Soriana gift cards, saying they had been distributed by a PRI-affiliated union, and it filed a complaint to electoral authorities. The party's presidential candidate, Andres Manuel Lopez Obrador, came in second.

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

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

Lopez Obrador said Tuesday that his team had detected irregularities at 113,855 polling places, and called for a total recount.

"This is a scandal. ... They bought millions of votes," Lopez Obrador said at a news conference, referring to the PRI. Lopez Obrador has refused to accept the preliminary vote tallies, saying the election campaign was marred by overspending and favorable treatment for Pena Nieto by Mexico's semi-monopolized television industry.

Lopez Obrador also refused to concede when he lost a razor-thin race in the 2006 presidential race and his followers went into the streets to protest for months.

Lopez Obrador said he would not accept the preliminary election results reported by the Federal Electoral 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.

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