Thousands of college-age demonstrators marched down Mexico City's main boulevard Saturday to protest a possible return of the old ruling Institutional Revolutionary Party, known as the PRI, which held Mexico's presidency without interruption from 1929 to 2000.

PRI presidential candidate Enrique Pena Nieto leads in polls ahead of the July 1 election, but he was heckled by young protesters during a recent appearance at a university. Students blamed him for a violent crackdown on protesters outside Mexico City in 2006. Later, some PRI members suggested the hecklers weren't really students, further enflaming passions.

In a move unusual for Mexico, the demonstrators did not carry banners for any of the other three candidates in the presidential race, instead shouting slogans against what they don't want, a return of the PRI, whose 71-year-rule was marked by repression, corruption and periodic economic crises.

Playing on the PRI's name, one banner read "I don't want a PRImitive Mexico." Another banner read: "We don't forgive or forget 70 years of corruption."

Pena Nieto has said he leads a new and reformed PRI, in which a younger generation has learned from, and paid for, the mistakes of the past.

While the march was led by a banner listing massacres and crimes allegedly committed or abetted by PRI governments since the 1960s, many of the marchers, like university student Fernanda Vargas, 26, are too young to have spent much time under PRI administrations; she was barely 14 when the last PRI president, Ernesto Zedillo, left office in 2000.

"I don't remember much, but my mother has told me about it, and we don't want to return to that," said Vargas, who is studying a masters' degree in public policy. She wasn't buying Pena Nieto's argument that the PRI has changed. "The PRI is selling itself as something new, but it really has the same old vices, the corruption."

Her mother, Guadalupe Barrera, 57, said she remembers the PRI years as a time when "you couldn't speak out, if you were in the opposition." Barrera, who only studied through grade school, said a PRI return "would be like a horror movie."

While Pena Nieto, 45, is the youngest of the candidates but he has struggled to gain support among university students, in part because of some missteps.

After the heckling incident earlier this month at posh private university, some of his supporters said the protesters weren't really students or had been manipulated by rival candidates. That led to about 130 of them to make a video posted on social media sites, in which they were seen holding up their student IDs and denying they had been manipulated.

The involvement of private university students in the forefront of Saturday's protests was unusual in Mexico's heavily class-conscious political sphere; the more working-class student bodies at public universities have played more of a role in the past.

Young people have had only a sporadic involvement in Mexican electoral politics the past. For example, in the failed 2006 presidential campaign of leftist Andres Manuel Lopez Obrador, older voters _ many of whom benefited from the ex-mayor's pension scheme _ formed the hard core of his support.