Mexican federal police clash with teachers during a protest against an education reform and the arrest of two of its leaders, in Oaxaca State, on June 19, 2016. At least six people were killed and dozens injured. (PATRICIA CASTELLANOS/AFP/Getty Images)

The long-simmering dispute between Mexico’s federal government and a radical arm of the country’s teachers union erupted into violence over the weekend, as riot police clashed with protesters in the southern state of Oaxaca, leaving at least six dead and more than 100 others wounded.

Teachers canceled classes in Oaxaca on Monday after the violence, where protesters threw rocks and  molotov cocktails and set vehicles ablaze. Witnesses  reported that police fired into the crowds...

The violence marked the bloodiest moment in a conflict that has intensified during the administration of President Enrique Peña Nieto. As part of Peña Nieto’s reform agenda, authorities overhauled the public education system, requiring mandatory testing for all teachers.

Teachers clash with Mexican Federal police in Oaxaca on June 20, 2016. (PATRICIA CASTELLANOS/AFP/Getty Images)

The National Coordinator of Education Workers,  the dissident faction of the national teachers union, has fought  those changes from the beginning by holding protests.  Its members, particularly those from the most aggressive branch in Oaxaca, called Section 22, have blocked roads, burned buildings, seized oil-distribution facilities and tried to  boycott last year’s mid term elections.

More recently, federal prosecutors accused union leaders of stealing public funds, prompting a new wave of roadblocks, bus stoppages and other civil unrest. In the past week, protesters also cut off access to the strategically important Salina Cruz oil refinery, which makes jet fuel.

The most intense clash occurred in the municipality of Nochixtlan, northwest of the capital city Oaxaca. Police on Sunday responded to another roadblock, which escalated into gunfire. According to published reports in the Mexican media,  rows of riot police faced off against masses of protesters, with buses blocking the road between them and plumes of black smoke rising from street fires.

The governor of Oaxaca, Gabino Cue, and the chief of the federal police, Enrique Galindo, told reporters Sunday evening that six civilians died.  They said an additional 53 civilians, 41 federal police and 14 state police officers were injured.

Leaders of Section 22 on Monday  put the death toll at  eight. They said 22 people had disappeared and 45 suffered gunshot wounds. The union officials said during a news conference that the victims included teachers, parents and others. Union officials called the violence a “massacre” and demanded the resignations of  Peña Nieto,  Cue and others.

Mexican authorities denied that any teachers were among the dead and said that youths and local merchants, who may have been involved in the protest, or bystanders at the bustling Sunday street market in Nochixtlan, were among those killed.

In an interview on Mexican radio, Galindo described the violence as an “ambush” against police by about 2,000 people who had surrounded them. Authorities blamed at least seven militant organizations, including various guerrilla groups, of infiltrating the protests and firing on police to ignite the chaos.

“We began to see that they brought molotov cocktails, powerful rockets. I have many police burned on their hands and feet, who lost fingers,” Galindo said.

Violence was reported in other parts of Oaxaca. People set fire to a federal police station in Huajuapan de Leon, northwest of Oaxaca city, on Sunday night. On Monday, teachers protested in Acapulco.

Mexico’s National Human Rights Commission said it would open an investigation into the deaths.

Some analysts blamed the federal government for the violence.

“They know the limitations of the Mexican cops. Despite that, they sent them in there with guns,” said Rodolfo Soriano-Nuñez, a sociologist who formerly worked in the federal education ministry. “The whole thing is a mess.”

Peña Nieto’s administration has made education reform — and wresting power from the teachers union — a priority. Authorities opposed how the union’s dissident factions could control decisions over state education budgets, teacher appointments and other administrative decisions. In addition to implementing the new testing program and firing about 5,000 teachers who would not participate, authorities dissolved a Oaxaca state education agency controlled by Section 22, arrested at least eight of its leaders and deployed federal police to ensure that teachers could take the required tests safely.

Opponents have argued that the federal government has ignored failing schools across the poor states in rural southeastern Mexico. They say that tests don’t adequately assess teachers’ skills and particularly punish those in poor, rural areas who have different educational backgrounds.

Aurelio Nuño Mayer, the education secretary, told The Washington Post earlier this year that 2016 “will be brutally intense, with a level of transformation that we haven’t seen in decades.”

Sunday’s violence has evoked memories of a decade ago, when teachers  clashed with police in Oaxaca. The 2006 unrest lasted for months, as strikes over teachers’ salaries morphed into other demands. The colonial cobblestone streets of Oaxaca city looked like a battleground, with barbed wire and barricades, and police eventually  drove out the protesters.

  As the conflict has dragged on,  all sides have been accused of corrupt behavior at the expense of students.

“There are no saints, there are no heroes, down there,” Soriano-Nuñez said.