Mexican teachers are protesting against education reforms which would force them to undergo controversial performance evaluations.
Six people were killed in Mexico's southern state of Oaxaca on Sunday when police clashed with thousands of protesters blocking streets in the town of Asuncion Nochixtlan.
More than 100 people including civilians and police agents were injured, while 21 people were detained.
According to the protesters the violence came after police used lethal weapons to disperse the demonstration. However, the head of Mexico's federal police, Enrique Galindo, denied the accusations.
Masked men who were not affiliated to the teachers union were behind much of the violence, shooting at officers and throwing petrol bombs at civilians, Galindo said.
"These kinds of radicalized protests generate violence," he said.
The demonstrators from the National Coordinator of Education Workers (CNTE) oppose education reforms which were introduced by Mexican President Enrique Pena Nieto in 2013.
The reforms will require teachers to undergo performance evaluations before being placed in public schools. Those opposed to the changes say the evaluations act as a cover for mass layoffs and are not a fair measurement of teaching ability.
The protesters have been blocking streets and disrupting traffic on a major highway connecting Oaxaca to Mexico city ever since some of the union leaders were arrested.
On Friday, state-run oil company Pemex warned that there might be a shortage of gasoline, diesel and jet fuel in the region, if the blockages persist.
Earlier on Sunday, 120 tanker trucks operated by Pemex were escorted by police due to the highway being blocked.