By Noe Torres and Mica Rosenberg
MEXICO CITY (Reuters) - Thousands of demonstrators protested in Mexico City on Saturday against opposition presidential candidate Enrique Pena Nieto, who is far ahead in polls and poised to lead the party that ruled Mexico for much of the 20th century back to power.
A contingent of mainly students, accompanied by groups of unionized workers and peasant farmers, held banners lambasting the Institutional Revolutionary Party, or PRI, and its candidate, Pena Nieto.
"I have a brain, I won't vote for the PRI," one banner read.
Demonstrators also held signs accusing Pena Nieto of being unfairly favored by television companies and of having been corrupt and repressive as governor of Mexico State, a post he held from 2005 until 2011.
"I am not with any party, but I am sick of so much corruption," said Eduardo Nolasco, a 22-year-old student.
"We are fed up of so many lies and of the hypocrisy of Pena Nieto and the media," added Isabel Leyva, a 53 year-old house wife who was accompanied by her daughter, a student.
Police said there were more than 40,000 protesters at the demonstration.
Pena Nieto's campaign team have vigorously denied accusations of corruption and say they have had no favoritism from TV media in the run-up to the July 1 election.
A telegenic 45-year old with a soap opera star wife, the PRI candidate also argues that his party has changed dramatically in the last decade.
The PRI held the Mexican presidency continuously from 1929 until 2000, during which time there were widespread accusations of vote rigging and violent crackdowns on protests.
It finally lost the presidency to the pro-business National Action Party, or PAN, a victory that was hailed as a triumph for democracy. The PAN held power in 2006 with the election of President Felipe Calderon.
But the PRI has regained support amid rampant drug violence and sluggish economic growth.
An opinion poll on Friday found that 45.5 percent of decided voters said they would cast ballots for Pena Nieto, giving him a lead of more than 20 points over the nearest of his rivals.
Calderon is prohibited constitutionally from running for a second term. The PAN's candidate Josefina Vazquez Mota was running a distant second at 25.1 percent in the poll.
Leftist candidate Andres Manuel Lopez Obrador was in third place with 23.4 percent support, according to the survey published by Milenio newspaper and conducted by pollster GEA/ISA.
Analysts said Pena Nieto could face political trouble if demonstrations against his party and candidacy spread.
"If we start to see constant and repeated confrontation and rejection of Pena Nieto, than this could have an effect," Ulises Beltran of the polling firm BGC said, adding that anti-Pena Nieto protests could help tighten the presidential race.
Protesting students booed Pena Nieto when he visited an upscale private university in Mexico City earlier this month. The demonstration on Saturday was promoted vigorously on social media sites, which are favored by young voters.
Other analysts said the protests would be unlikely to swing the race.
"The young, upper middle-class university students (active on social media) are not that important in population terms, but they are a group that can make a lot of noise," pollster Roy Campos said.
(Reporting by Mica Rosenberg, Noe Torres and Ioan Grillo; Writing by Ioan Grillo; Editing by Paul Simao)