Mexico City (CNN) -- Protesters marched and packed the main square of Mexico City on Sunday, slamming the candidate widely seen as the front-runner in next month's presidential elections.
Enrique Peña Nieto, the Institutional Revolutionary Party candidate, leads in polls. His party -- better known as the PRI -- ruled for some 70 years, before losing the presidency in 2000. Demonstrators say his winning would be a step back for democracy in Mexico.
"I'm here because I'm really angry with what's going on in my country. I think it's not fair," said Adrian Garcia, 34, a protester.
He wore a white T-shirt with the words "Yo Soy 132," ("I am 132") printed on it, referencing a nascent protest group that Peña Nieto inadvertently helped to create.
After students jeered the candidate at a recent campaign event, Peña Nieto dismissed the demonstrators as outsiders, dragged there by political operatives to cause commotion.
Three days later, a YouTube video featured 131 students flashing their university ID cards, saying "We are students from Ibero. ... Nobody forced us to do anything." The 11-minute video went viral.
Posts promoting protests throughout the country on social media have used it as a jumping off point, using the phrase "#YoSoy132" on Twitter and Facebook.
But Sunday's protests in Mexico City attracted more than just students -- teachers, unions and various political groups were also there.
The demonstrations were timed to coincide with the last scheduled televised debate before the presidential election, which is set for July 1. Similar, though smaller, rallies were held in several other Mexican cities.
Garcia, the protester, said he voted for leftist candidate Andres Manuel Lopez Obrador in the last election. Lopez Obrador never conceded after officials declared Felipe Calderon the winner of Mexico's election in 2006 and has been known to refer to himself as "the legitimate president of Mexico."
The former Mexico City mayor is running again.
"They are touching on a fundamental theme that has to do with the pretension of dominating the country through the almost absolute control of the media," Lopez Obrador said recently about the protesters.
He and other opponents have accused Peña Nieto of receiving preferential treatment from the media since before the campaign began. The candidate, who was governor of the state of Mexico from 2005 to 2011, has denied those accusations and said he respects the youth protests against him.
CNNMexico.com and Miguel Marquez contributed to this report.