Tens of thousands of people marched in Mexico's capital on Saturday to protest Enrique Pena Nieto's apparent win in the country's presidential election, accusing his long ruling party of buying votes.

The protesters were angered by allegations that Pena Nieto's Institutional Revolutionary Party gave out bags of groceries, pre-paid gift cards and other goods to voters ahead of July 1 national elections.

The students, unionists and leftists in the march carried signs reading, "Pena, how much did it cost to become president?" and "Mexico, you pawned your future for 500 pesos." Mexico City officials put the size of the crowd that reached its central Zocalo plaza at 50,000.

"The fraud was carried out before (the election), buying votes, tricking the people," said Gabriel Petatan Garcia, a geography student who carried a sign in Finnish.

Protesters also carried signs in English, Japanese, French, German and other languages to call the attention of the international press.

Pena Nieto, a youthful, 45-year-old married to a soap opera star, won last Sunday's election by 6.6 percentage points, according to the official count, bringing the PRI back to power after 12 years in opposition. The party had ruled Mexico for 71 consecutive years, with what critics say was the help of corruption, patronage and vote fraud.

PRI officials deny the vote-buying charge and say the vote was free and fair.

The final vote count had Pena Nieto getting 38.21 percent support, leftist Andres Manuel Lopez Obrador of the Democratic Revolution Party with 31.59 percent, and Josefina Vazquez Mota of the conservative National Action Party with 25.41. The small New Alliance Party got 2.29 percent.

The final vote count must be certified in September by the Federal Electoral Tribunal. The tribunal has declined to overturn previously contested elections, including a 2006 presidential vote that was far closer than last Sunday's.

Accusations of vote-buying began surfacing in June, but sharpened later when people rushed to grocery stores on the outskirts of Mexico City to redeem pre-paid gift cards worth about 100 pesos ($7.50). Many said they got the cards from PRI supporters before the elections.

Lopez Obrador said millions of voters had received either pre-paid cards, cash, groceries, construction materials or appliances.

Some marchers covered the heads of statues with plastic shopping bags from Soriana _ the supermarket chain where the gift cards were redeemable _ to underline their protest.

"We have to come out in the streets to denounce that the PRI bought votes, and there were people who sold them," said 32-year-old psychologist Raquel Ruiz.

Some protesters felt that overturning the election result would be difficult at this point, while others thought there were judicial means to still prevent Pena Nieto from assuming the presidency.

Lopez Obrador said he will file a formal legal challenge to the vote count in electoral courts in the coming days based on the allegation that PRI vote-buying illegally tilted millions of votes.

Simply giving away such gifts is not illegal under Mexican electoral law, as long as the expense is reported to electoral authorities. Giving gifts to influence votes is a crime, though is not generally viewed as grounds for overturning an election.

Leonardo Valdes, the president of the Federal Electoral Institute, has said he doesn't see any grounds for overturning the results but that an investigation into the gift cards had been launched.

PRI spokesman Eduardo Sanchez said earlier this week that the gift-card event had been "a theatrical representation" mounted by the left. Sanchez claimed supporters of Lopez Obrador took hundreds of people to the stores, dressed them in PRI T-shirts, gave them gift cards, emptied store shelves to create an appearance of panic buying, and brought TV cameras in to create the false impression that the PRI had given out the cards.

Cesar Yanez, the spokesman for Lopez Obrador's campaign, denied the PRI accusation.