Four people were killed and dozens injured when police clashed with thousands of teachers blocking roads in a protest in southern Mexico on Sunday, leaving some officers with bullet wounds.
The radical union, which goes by the initials CNTE, is opposed to the mandatory testing of teachers as part of the government's sweeping education reform and is also protesting the arrest of union leaders on money laundering and other charges.
Sunday's clashes involved federal and state police.
Police launched tear gas to disperse the teachers and authorities denied reports that officers were wielding lethal weapons to break up the protest in Asuncion Nochixtlan, a town in Oaxaca state.
The National Education Workers Coordinator (CNTE) union has been leading protests in Oaxaca for days against an education reform and the arrest of two of its leaders.
"There are 4 (people) dead and 46 injured," Oaxaca state health secretary Hector Gonzalez told AFP.
In a statement, federal police said protests have been the most aggressive in Nochixtlan, north of Oaxaca City, where protesters even took a police officer hostage.
A state official, who was not authorized to speak to the press and requested anonymity, said a Oaxaca state police officer was killed.
The National Security Commission said in a statement that 21 federal and state police were injured, including "several of them from firearms."
The commission said the officers were not carrying guns or truncheons, but Mexican media showed pictures of police holding rifles.
"We know that the shots from firearms come from unknown people and unconnected to the blockades, who are firing against the population and the authorities to trigger clashes," it said.
A municipal police officer, who requested anonymity, said more than 30 people were arrested.
Clashes lasted hours in the municipality of San Pablo Huitzo, also north of the state capital. And clashes were continuing Sunday evening outside Oaxaca City itself, where protesters burned federal police installations.
On Saturday night, police removed protesters who were blocking a major highway on the isthmus of Tehuantepec.
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 in Oaxaca City, protesting teachers haves 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.
Ten years ago, the teachers started a six-month takeover of Oaxaca that didn't end until police stormed the barricades.
- AP, AAP