Mexico's Pena Nieto hounded online by youth groups By Mica Rosenberg and Gabriel Stargardter MEXICO CITY (Reuters) - Long before the Mexican presidential election officially began, front-runner Enrique Pena Nieto recruited an army of supporters on the Internet that dwarfed those of his rivals. But with less than three weeks to go before the July 1 vote, some of the fiercest resistance to his candidacy is being waged online. Organizing on the Web, young activists have launched protests against Pena Nieto's Institutional Revolutionary Party (PRI), which ruled Mexico for most of the 20th century despite accusations of corruption and violent repression of dissent. Taking power in 1929, the PRI was ousted by the conservative National Action Party (PAN) in 2000 and has pinned its hopes of a return to office on the telegenic Pena Nieto, 45. Marketed as the new face of the party, Pena Nieto set out to connect with first-time voters - around a sixth of the electorate - who have fewer memories of the PRI's past. When Pena Nieto stood down as governor of the State of Mexico in September, he had more fans on Facebook than the other main presidential hopefuls and President Felipe Calderon combined. Even today, he still has six times as many followers as his closest rival, leftist Andres Manuel Lopez Obrador. However, the sudden anti-Pena Nieto movement contends the centrist PRI is still dominated by the party's old guard, often working closely with Mexico's most powerful broadcasters. The dissent has chipped away at Pena Nieto's support among young voters and breathed new life into the campaign of Lopez Obrador, who narrowly lost the last election in 2006. Most pollsters say Pena Nieto has a double-digit lead and predict he will win, especially after he emerged from a final televised debate on Sunday largely unscathed. Around a fifth of voters are still undecided in most polls. Nevertheless, in just over a month, support for Pena Nieto among 18- to 29-year-olds has slipped by 9 percentage points and jumped by nearly the same amount for Lopez Obrador, according to the Consulta Mitofsky polling firm. "Even if Pena Nieto wins, at least he'll know it won't be as easy for him as it always was," said Gonzalo Ochoa, 36, one of tens of thousands at an anti-PRI march on Sunday in Mexico City. A victorious Pena Nieto could face trouble in government if the groups mobilizing against him keep up the pressure. We are first working toward July 1 but also developing an agenda for after. Everyone is fired up," said Ivan Hernandez, a protest organizer in Guadalajara, Mexico's second-biggest city. The buzz began last month when a crowd of students at Mexico City's private Ibero-American university heckled Pena Nieto and made videos of his uncertain reaction, then posted them online. One grainy cell phone clip from the event showing the usually composed candidate looking worried and trying to escape the protest has been viewed nearly a million times on YouTube. His campaign suggested the hecklers weren't really students, prompting 131 people to hold up their university cards in a video, giving birth to the movement "Yo soy 132" or "I am 132." Foremost among the anti-PRI groups, "Yo soy 132" helped push for the last presidential debate to be broadcast on more national channels after the first face-off had to compete with a top-flight soccer match and a popular variety show for air time. Just before the Ibero protest, 38 percent of voters had a positive impression of Pena Nieto and only 18 percent had a negative one. The same pollster, Mitofsky, reported this week that the gap has narrowed to 35 percent to 25 percent. The scale of the anti-Pena Nieto movement is restricted by low Internet coverage in Mexico, where more than half the population lives in poverty. Only 36 percent of Mexicans are online, compared to 77 percent of U.S. households. But usage is growing fast, rising by 14 percent last year. Communications student Alejandro Gomez said it will take word of mouth to get the message off-line and onto the street. Even if everyone doesn't have the Internet, the information comes from there. I find out about it and tell my mom and she tells her neighbor who doesn't have a computer," said Gomez, 21. The reaction has spurred other acts of defiance against the PRI candidate, taking the shine off his slick campaign. This week, Internet activist group Anonymous Hispano posted personal details of Pena Nieto online including what it claimed were his phone numbers. On Tuesday, a crowd ran after his convoy in Puebla state, heckling him and hitting his car. Pena Nieto has responded by saying he welcomes differences of opinion, but a parallel backlash against Mexico's two main broadcasters has helped to add fuel to the flames of protest. Televisa and rival TV Azteca control 95 percent of the television market, and there has long been suspicion that they have taken money to support powerful politicians. An article last week by British newspaper The Guardian included documents that appeared to show Televisa sought to raise Pena Nieto's profile and discredit Lopez Obrador in 2006 has created a buzz in Mexican online social media. The enthusiasm for taking on Pena Nieto has spawned other movements looking to replicate the media impact of "Yo soy 132." But therein may also lie a problem for the opposition. "On social networks it is various choruses competing for who can be louder," said Jose Carreno, a Mexican media expert. The problem is they end up neutralizing each other. Additional reporting by Lizbeth Diaz; Editing by Dave Graham