Fifty thousand protesters marched through Mexico City on Saturday to denounce Enrique Pena Nieto's victory in the July 1 presidential election.
The demonstrators accused Mr Pena Nieto's Insitutional Revolutionary Party (PRI) of having "bought" votes by handing out millions of bags of groceries, pre-paid gift cards and other goods.
Students and trade unionists carried banners reading: "Pena, how much did it cost you to become president?" and "Mexico, you pawned your future for 500 pesos (£25)."
Marchers held signs in English, Japanese, French, German and even Finnish as well as Spanish in a bid to grab the attention of the international press.
Critics of the PRI raised accusations of vote-buying in June, but the issue gained momentum when supermarkets in the capital were flooded with shoppers seeking to redeem pre-paid gift cards in the couple of days following the presidential election.
Many said they had been given the cards by PRI supporters - though the PRI in turn has accused left-wing candidate Andres Manuel Lopez Obrador of dressing his supporters in PRI outfits and handing out the cards, then staging the run on the shops.
A final verification of the vote, which saw Mr Pena Nieto beat Mr Lopez Obrador by 38 to 32 per cent, will take place in September.
The Federal Election Institute has pledged to investigate the gift-card claims, but the overturning of the result is unlikely as political parties are free to give out gifts before elections provided that they do not explicitly seek to influence votes with them.