Tens of thousands of people marched through Mexico City on Saturday to protest against the result of last Sunday's presidential election.

The demonstrators, who included trade union activists, leftists, students  and anarchists, accuse the Institutional Revolutionary Party (PRI) of Enrique Pena Nieto, who won the election, of widespread vote-buying.

“Out Pena, Mexico without the PRI,” the protesters changed as the crowd marched down a main boulevard to Zocalo square in the center of the capital.

Municipal officials estimated the number of participants at 50,000.

Election result certified

The march came a day after Mexico's election commission certified Pena Nieto's victory, saying he had beaten his nearest opponent, leftist candidate Andres Manuel Lopez Obrador, by almost seven percentage points.

The country's electoral tribunal has until September to hear any official complaints about the vote, before officially naming the next president. Unless it decides to overturn the result of the vote, Nieto will take office in December.

The tribunal has declined to overturn the results of previous elections that faced legal challenges, including the 2006 vote, in which Obrador also finished second, but by a much narrower margin.

Obrador has refused to concede defeat in this year's election and on Saturday he called on the National Action Party (PAN) of outgoing President Felipe Calderon to join forces with him in launching a legal challenge to the result.

Constitutional limitations prevented Calderon from running for a third term - the PAN candidate finished third in Sunday's vote.

Obrador said he had no involvement in Saturday's march, which was largely organized using social network websites like Facebook or Twitter. This may have been part of the reason that most of the marchers appeared to be under 30.

Gift-card controversy

The specific allegations of vote-buying against Pena Nieto's party stem from a rush on a chain of supermarkets, by consumers wielding gift cards they said they had received from the PRI. The party has denied any wrongdoing and the supermarket chain in question has said the cards were part of one of its promotions.

Elections officials said they were looking into the matter, although on the surface, there appeared to be no grounds for overturning the results.

Pena Nieto, 45, was elected on a platform of reforming labor laws in an effort to increase economic growth and increasing spending on law enforcement to reduce the country's high murder rate.

His election returned the PRI to the presidency after an absence of 12 years. Prior to that, the party had ruled Mexico for seven decades, during which it faced allegations of vote rigging and repressing popular dissent.

pfd/ch (Reuters, AP, AFP)