OAXACA, Mexico — Mexican authorities and protesters on Monday traded accusations of responsibility for weekend clashes that left at least six people dead and more than 100 wounded in the restive southern state of Oaxaca.

Federal Police Chief Enrique Galindo, speaking on local Radio Formula, said few teachers were involved in the violence and attributed it to other, unspecified “radical groups.”

However the radical teachers’ union involved in the protests denied that and alleged that police infiltrators were to blame.

The clashes are the latest flashpoint in an ongoing battle for control of public education in Oaxaca, where the union is resisting government attempts to implement national education reforms passed under President Enrique Pena Nieto.

Galindo said things initially went smoothly Sunday when officers moved to reopen the highway around 7 a.m. after it had been blocked by protesters. Traffic flow resumed for about two hours following dialogue between unarmed police and demonstrators from the National Coordinator of Education Workers, or CNTE.

But later the crowd swelled to about 2,000 protesters, some of them armed with gasoline bombs and powerful fireworks, Galindo added. When police confirmed gunshots, he ordered armed police to move in.

“It was a radical change of scene,” Galindo said. “It was practically an ambush.”

He reported that seven officers suffered bullet wounds, others had serious burns on their hands and feet and some lost fingers.

Six people were killed and more than 100 were wounded before police pulled back, he said, adding that “staying in Nochixtlan would have brought more serious consequences.”

Late Sunday, Oaxaca state Gov. Gabino Cue said all the dead were civilians and two had ties to the CNTE union.

On Monday, the CNTE said eight people had been killed and 20 more were missing. It said the dead comprised teachers and members of other social groups that support them.

But the union denied the presence of “radical groups” and said police had infiltrated their movement. It demanded Cue resign and blamed him and national Education Secretary Aurelio Nuno for the upheaval.

“This movement is not going to stop,” said union member Juan Garcia.

Galindo said he did not know if police had fired any of the fatal gunshots. Video filmed by The Associated Press showed at least one officer firing a gun several times, though it was unclear if he was a federal or state agent.

About 100 to 150 protesters maintained the roadblock on Monday in Nochixtlan, where about a half-dozen burned vehicles littered the area and there were no police in sight. Protesters were allowing people to pass on the highway but continued to block commercial traffic.

Galindo said there were a number of blockades around the state and officials would try to resume talks with the teachers.

The federal attorney general’s office said it would assist Oaxaca prosecutors in investigating how the shooting broke out.

Several thousand teachers and activists marched in Oaxaca city, chanting “Murderers!”

The protesters demanded the federal government sit down to negotiate the educational reform; to date teachers who have refused to take evaluation tests have been fired. The radical teachers’ union in Oaxaca says local conditions in the heavily indigenous state should be taken into account in the testing.

Despite the largely peaceful atmosphere in Monday’s march, a group of masked men tossed rocks and powerful fireworks at public buildings and police.

On Monday afternoon, Oaxaca city’s graceful main plaza was hidden under a sea of tarps and small tents set up by protesters. Garbage sat in heaps around the perimeter, and the cafes that surround the square and would normally be packed with tourists were empty.

Anarchist graffiti covered the facades of historic buildings on streets radiating out from the zocalo. The tall glasses windows of a bank a block from the square were all broken and the ATMs just inside the front door were vandalized and spray painted with anarchist logos.

Mexico’s National Human Rights Commission reported that journalist Elpidio Ramos Zarate of the newspaper El Sur del Istmo in Juchitan de Zaragoza, Oaxaca state, was killed Sunday, but it was not immediately clear if it was related to the protests.

Ramos Zarate had reportedly been taking pictures of some looters in Juchitan cleaning out store shelves. The Inter American Press Association quoted the news manager of Ramos Zarate’s paper as saying he had been threatened by masked men who demanded he not cover the protests and looting.

Teacher protests have been going on for years in Oaxaca. A decade ago, the union launched a six-month takeover of central Oaxaca city that only ended after police stormed the barricades.

Mexico’s education reform aims to wrest control over struggling schools from unions that have often had sole power over hiring, firing, promotions and budgets. The CNTE strenuously opposes a new system of teacher evaluations and rules allowing for educators to be dismissed if they miss too many schooldays.

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.