TIXTLA DE GUERRERO, Mexico (AFP) – Rebellious teachers and protesters angry at President Enrique Pena Nieto’s administration burned ballots and blocked polling stations on Sunday in efforts to thwart midterm elections.

The protests in the impoverished southern states of Oaxaca and Guerrero occurred despite the deployment of federal police and troops to ensure people can cast their votes across the country.

A radical teachers union is putting pressure on Pena Nieto to withdraw a landmark education reform aimed at improving the country’s lackluster school system.

The elections are a major test for Pena Nieto, whose Institutional Revolutionary Party (PRI) and its allies expect to maintain a congressional majority, despite the protests and political scandals.

Despite the rocky campaign season, authorities have voiced confidence that the elections for 500 members of the lower chamber of Congress, around 900 mayors and nine governors, will not be stopped.

 Some of the protesters have focused their anger on the collusion between gangs and politicians, especially in the drug violence-plagued state of Guerrero.

In Tixtla, Guerrero, hundreds of people wielding sticks protected a polling station. Election opponents arrived and the two sides threw rocks at each other but no injuries were reported.

While a police helicopter hovered overhead, there were no federal forces on the ground.

Local PRI officials said only two of 13 polling stations opened in Tixtla, home of the teacher training college attended by 43 students who were allegedly killed by a police-backed drug gang last year.

Relatives of the 43 young men and masked students snatched election material and burned it.

“As long as they don’t deliver our sons, there won’t be elections,” said the father of one of the 43 students, whose parents refuse to believe they are dead and have vowed to prevent the elections.

In Oaxaca, a bastion of a radical teachers union, protesters burned ballots, ballot boxes and cardboard voting screens at around 20 polling locations, authorities said.

A bus was set on fire on a federal highway, police said, while in the mountain town of Huautla de Jimenez teachers cut down trees and place rocks on the road to prevent federal forces from coming.

Thousands held a protest in Oaxaca’s capital.

“The possibility that social violence could have a role in limiting the vote, affecting the results, is unprecedented in Mexico’s modern history as a democracy,” said Javier Oliva, security expert at the National Autonomous University of Mexico.

Other violence is also a concern in regions like Guerrero plagued by organized crime.

At least 10 people were killed on Saturday in Guerrero when rival factions of a self-defense militia clashed in the village of Xolapa, though authorities suggested the fight was linked to an internal feud and not the elections.

In addition to the protests, at least four candidates were murdered in the run-up to the election, including three in Guerrero and Michoacan.

The deployment of federal forces followed daily protests this week spearheaded by the CNTE, which stormed election offices, burned thousands of ballots and ransacked headquarters of political parties in Guerrero, Oaxaca and Chiapas states.

The violence has overshadowed a campaign that could yield no changes in Congress but could still make history in the industrial state of Nuevo Leon.

There, Jaime “El Bronco” Rodriguez is riding a wave of discontent against corrupt politicians and could become the first ever independent to be elected governor.