Parents of 43 missing students clashed with riot police in southern Mexico while radical teachers blocked an airport and ransacked an electoral office elsewhere in new protests against upcoming elections.

Felipe de la Cruz, a spokesman fore the parents, said police had blocked buses carrying more than 100 relatives of the missing young men and fellow students from the Ayotzinapa teacher college who were heading to Guerrero state's capital, Chilpancingo.

Masked protestors standing on top of a tunnel threw rocks at dozens of police on the road below, while officers responded with tear gas. Two students and an officer were hurt, according to television images.

Parents of the students and teachers from a dissident union have vowed to block Sunday's midterm vote.

The relatives are angry at the handling of the case. Authorities say corrupt local police abducted the 43 students in September and handed over to a drug gang, which slaughtered them.

Teachers, meanwhile, want the government to cancel an education reform and have pressed on with protests even though the government suspended a main component of the legislation, mandatory testing for teachers, last Friday.

In the neighboring state of Oaxaca, some 300 members of the CNTE teacher union used trucks to block access to an international airport, causing several flight cancellations.

In the southern state of Chiapas, several teachers stormed the offices of the National Electoral Institute in Tuxtla Gutierrez, burning furniture and breaking windows.

Earlier this week, other teachers stole and burned thousands of ballots in Oaxaca and Guerrero, but authorities say new ones would be reprinted.

Mexicans will vote Sunday for 500 members of the lower chamber of Congress, mayors in nearly 900 municipalities and governors in nine states.