Read a joint statement about this story from the Guardian and Televisa here.

Thousands attended a demonstration on Sunday against leading Mexican presidential candidate Enrique Peña Nieto, alleging mass media bias in his favour.

Marches in the capital and other cities took place a few hours before the second and final official debate between the presidential candidates. Polling day is in three weeks.

The demonstrators in the capital ranged from longstanding radical leftwing groups, whose members dressed in identical T-shirts and blared chants through a loudspeaker, to the more spontaneous students who have given the anti-Peña Nieto movement its dynamism in recent weeks.

The students, frenetic users of social networking sites, have also focused a critical spotlight on what they claim is chronic manipulation of coverage by the two commercial TV networks that control almost all free television programming in Mexico, particularly the biggest, Televisa.

“We Want a Real President, not a Telenovela Actor,” read one banner, referring to charges that the young, good looking Peña Nieto built his popularity with the help of a soap opera hero image promoted by Televisa. Another read, “The Revolution Will Not Be Televised” to emphasise the allegation that the network initially downplayed the movement’s size and sincerity.

“Not everybody has Facebook and Twitter, so it’s important to show we cannot be ignored,” said Ivan Rosas, 24, a psychology student. “The television networks are going to have to start realising that we are the ones making the news, not them.”

A victory for Peña Nieto would return the Institutional Revolutionary party (PRI) to power 12 years after it lost the presidency it had held for seven decades. Most opinion polls put Peña Nieto more than 10 points ahead of his nearest rival, the leftwing candidate Andrés Manuel López Obrador. His lead was around 20 points a few weeks ago.

The Guardian’s revelation of documents suggesting that Televisa sold favourable coverage to Peña Nieto when he was governor of the state of Mexico, and developed a dirty tricks campaign against López Obrador ahead of his first bid for the presidency in 2006, have intensified the debate. Televisa and the PRI have suggested that the documents are fake.