The ruling party of Mexican President Enrique Peña Nieto retained control of the country's congress Sunday, in elections marred by isolated violence, and despite widespread criticism of his administration, according to preliminary results. Peña Nieto’s Institutional Revolutionary Party (PRI) is expected to win about 30 percent of the vote, according to Agence France-Presse (AFP).

The country held elections for nine state governorships, and over 1,000 state and municipal posts. Mexico’s election cycle was marred by frequent violence, including the deaths of at least seven candidates and nine campaign officials, which were blamed on drug-related violence, Reuters reported. Peña Nieto’s government has undertaken a long-running and bloody campaign against the country's drug cartels.

On Saturday, protesters burned ballots and other election material to protest the kidnapping of 43 students last year in the city of Ayotzinapa. The students were declared dead by Mexican authorities in November.

Dissident teachers' unions also raided campaign offices and burned ballots, threatening to block the vote. Their demands included higher wages and an end to teacher testing.

A vigilante battle near the city of Acapulco also left 13 dead on Saturday, according to Mexico’s human rights commission.

Peña Nieto reportedly deployed authorities to guard polling stations amid the violence. In total, 40,000 police and federal troops were sent to safeguard voters across southern Mexico, and officials said that 99 percent of all ballot stations remained open despite the violence.

Exit polls indicated that Jaime Rodriguez Calderon, known as “El Bronco,” is set to become the first independent governor of a Mexican state, since a 2014 electoral reform allowed candidates to run independently.

"This will be a citizens' government in Nuevo Leon. Nuevo Leon will be the beginning of a second Mexican revolution," Rodriguez told supporters after a poll showed him running six points ahead of the leading party candidate in the northeastern state, AFP reported.

Peña Nieto has faced a spate of corruption accusations, amid a widespread sense of disappointment in his government’s perceived failure to tackle drug crime and lawlessness, mainly in the country’s rural areas. A drug cartel linked to the kidnapping of the students in Ayotzinapa allegedly acted in collusion with local police.

In the last election, the PRI, along with its coalition partners -- the Green Party and the New Alliance -- had won just over half of Mexico’s lower house seats.