Violent clashes between police and members of a radical teachers' union who had blockaded roads in southern Mexico on Sunday left six people dead and more than 100 injured, officials said.

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.

In Sunday's clashes in the southern state of Oaxaca, 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.

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.

Cue said that all the dead were civilians, with two having ties to the CNTE union. A state official had previously said a state police officer was killed but it turned out the person was a civilian.

Earlier Sunday, Mexico's federal government released a statement saying 21 federal police had been wounded, three of them by gunfire, and that its agents who participated in the operation were not carrying guns.

"The attacks with guns came from people outside the blockades who fired on the population and federal police," it said.

But footage filmed by The AP shows at least one police officer firing a gun several times, though it was unclear if he was a federal or state agent.

Late Sunday night, Galindo acknowledged that he had sent in some officers with guns after agents came under fire.

"The police obligation is to protect the population," he said.

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.

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.

Ten years ago, the teachers started a six-month takeover of Oaxaca that didn't end until police stormed the barricades.