Tens of thousands of protesters 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 accused Pena Nieto's Institutional Revolutionary Party of giving out bags of groceries, pre-paid gift cards and other goods to voters ahead of the 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 carried signs in many 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 almost 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, allegedly with the help of corruption and vote fraud.

A final vote count showed centrist Pena Nieto getting 38.2 per cent support, Andres Manuel Lopez Obrador of the progressive Democratic Revolution Party with 31.59 per cent, and Josefina Vazquez Mota of the rightist National Action Party with 25.41. The small New Alliance Party got 2.29 per cent.

Investigation into gift cards launched

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 in July when people rushed to grocery stores on the outskirts of Mexico City to redeem pre-paid gift cards worth about 100 pesos ($7.60 Cdn). 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.

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.