Mexico City: At least three persons were killed in violent clashes between teachers opposing an education overhaul, and security forces in Mexico's Oaxaca state, officials said. In a statement, the state government confirmed the deaths in the clashes on Sunday in Asuncion Nochixtlan with 45 others injured. The clashes started when federal and state law enforcement officers moved in to clear a roadblock set up by members of Local 22 of the teachers union on the Oaxaca-Puebla federal highway in that municipality. The clashes lasted more than two hours and left dozens injured by stones and other projectiles and firearms. Oaxaca authorities said at least six police officers were injured by gunfire, one of whom suffered head trauma, and 21 people were arrested. The teachers torched a truck to prevent officers from advancing, regrouped after being dislodged and set up a new roadblock in the same area. They also overturned official vehicles. The teachers union, a powerful labour organisation that has around 200,000 members, opposes the 2013 education overhaul implemented by President Enrique Pena Nieto's administration. The union contends that the teacher evaluation process introduced by the overhaul is punitive and fails to account for regional differences in educational methods.