After tens of thousands of protesters in Mexico City demonstrated Saturday against President-elect Enrique Peña Nieto's victory in the July 1 elections, the leftist opposition party of runner-up candidate Andres Manuel Lopez Obrador has said it will appeal the results.

We'll go to the Electoral Court in the coming days, within the appropriate time frame, said Jesus Zambrano, chairman of the center-left Party of the Democratic Revolution (PRD).

Mexico's presidential elections have been marred by allegations of fraud and vote-rigging, with most of the criticism leveled at Peña Nieto and his Institutional Revolutionary Party (PRI).

Lopez Obrador -- who has refused to concede -- and his supporters have accused the PRI of overspending on campaigning, buying votes and receiving favorable biased media coverage.

In particular, the PRI has been accused of handing out $5.2 million worth of gift cards for the Mexican grocery store chain Soriana to voters before the election.

A partial recount last week of roughly half the votes upheld Peña Nieto's victory over Lopez Obrador by a margin of seven percentage points, but the new results did little to assuage frustration with the electoral process among many Mexican voters.

The PRI threatens many people and buys others with a couple of tacos, Manuel Ocegueda, a 43-year-old shop worker who took part in the protests Saturday, told the Reuters news agency.

Estimates place the amount of protesters gathered in Mexico City's Zocalo Square on Saturday at around 50,000, many of them students, labor unionists and leftists who support Lopez Obrador.

The scene was reminiscent of the aftermath of Mexico's 2006 elections when Lopez Obrador was narrowly defeated by little more than half a percentage point by National Action Party (PAN) candidate Felipe Calderon. He staged months of mass public protests amid allegations of fraud.

During the 2006 protests, Lopez Obrador held a symbolic inauguration ceremony and proclaimed himself the legitimate president of Mexico.

The PRD candidate shied away from the recent protests, but has said he will mount a legal challenge to the election results based on evidence of vote-buying by the PRI.

The PRI responded to the allegations of vote-buying, claiming that it was an elaborate scheme orchestrated by the PRD in which people posed as PRI campaigners and handed out gift cards to frame the party.

Peña Nieto is expected to take office in December.