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OAXACA, Mexico — Violent clashes between police and members of a radical teachers' union on Sunday left six people dead and more than 100 injured, officials said.

The violence erupted as anti-riot police moved in to dislodge protesters blocking a highway in the southern state of Oaxaca.

The teachers from the National Coordinator of Education Workers, or CNTE, are opposed to the mandatory testing of teachers as part of Mexico's sweeping education reform and are also protesting the arrest of union leaders on money laundering and other charges.

A riot police officer walks past a burned truck carrying chickens after clashes with teachers during a protest in Nochixtlan, Mexico, on Sunday. STRINGER / Reuters

In Sunday's clashes, protesters threw stones and Molotov cocktails, and burned vehicles, while Associated Press journalists saw riot police firing on protesters. Clashes took place in several municipalities in Oaxaca, but the most violent were in Nochixtlan, north of the state capital also called Oaxaca.

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The violence erupted as anti-riot police moved in to dislodge protesters blocking a highway in the southern state of Oaxaca. Television footage showed chaotic scenes of men running from police as gun fire rang out.

In a late-night press conference, Oaxaca state Gov. Gabino Cue, accompanied by Federal Police chief Enrique Galindo, raised the death toll from the clashes in Nochixtlan to six. They said 53 civilians, 41 federal police agents and 14 state police agents were injured.

Galindo said masked individuals who were not affiliated to the union were behind much of the violence, lobbing Molotov cocktails and shooting at police and civilians.

"These kinds of radicalized protests generate violence," he added.

Teachers and riot police clash in the town of Nochixtlan, Mexico, on Sunday. STRINGER / Reuters

Cue said that all the dead were civilians, with two having ties to the CNTE union.

Clashes were continuing Sunday night outside of Oaxaca city and in the municipalities of San Pablo Huitzo and Santiaguito, where protesters had burned federal police installations.

Over the past week, unionized teachers have blockaded streets, a shopping mall and even train tracks in the western state of Michoacan. They have also forced some bus lines to cancel trips to Oaxaca, which is a popular tourist destination, and blocked a highway on the isthmus of Tehuantepec. And in Oaxaca city, protesting teachers have set up an encampment in the city's main square.

Mexican federal police clash with teachers in Oaxaca State on Sunday. PATRICIA CASTELLANOS / AFP - Getty Images

Federal prosecutors accuse union leaders of setting up an illegal financial network to fund protests and line their own pockets. They allege the scheme operated in 2013-2015, when the union effectively controlled the payroll of Oaxaca's teachers.

Following the arrest of some if its top leaders, the union called for a revolt against Mexico's government.