Members of a radical Mexican teachers' union have broken into or blockaded electoral offices in the southern state of Guerrero amid vows to bloke Sunday's midterm elections.

The country's top electoral official said Monday's protests probably represent the biggest threat to the election, even more than drug cartel violence.

National Electoral Institute head Lorenzo Cordoba says teachers destroyed furnishings at two of the offices. And he says that teachers' opposition could force authorities to move polling places to alternate locations. The teachers blockade several other offices, forcing the electoral institute to suspend work in parts of Oaxaca state.

The teachers are protesting an education reform that mandates teacher testing.

Over the weekend the government appeared to bow to their demands and drew criticism for temporarily suspending testing of teachers.