MEXICO CITY—Thousands of students are joining a growing street-demonstration movement to protest Mexico's top two television networks, which they accuse of colluding to secure the leading presidential candidate's election this summer.

Across the capital, as well as several other cities, university students in recent days have rallied against what they say is an effort by the country's television duopoly—Grupo Televisa SAB and TV Azteca SAB—to back the candidate of Mexico's former ruling party.

The Institutional Revolutionary Party, or PRI, ruled Mexico with a velvet hand and an iron fist for 71 years until it was voted out of power in 2000, but is now favored to regain power.

While the students, who were set to march again Wednesday night, have made the media coverage their rallying cry, they also are airing concerns about the return of the PRI—the party of their parents and grandparents.

The marches signal the first apparent threat to PRI candidate Enrique Peña Nieto's expected victory during an otherwise muted campaign. Televisa, which at first largely dismissed the protests as the work of leftist radicals, has now had to report them on its nightly newscasts.