Violence is threatening Mexico's midterm elections, with radical teachers attempting to block Sunday's voting.

The elections for Congress, governorships and mayorships have already claimed the lives of three candidates, one would-be candidate, and at least a dozen campaign workers or activists.

But unlike the years from 2010 to 2012, when the violence appeared to come largely came from drug cartels, radical movements now pose the greatest threat.

Unidentified assailants and radical teachers on Friday burned or attacked the offices of five political parties in the southern states of Chiapas and Guerrero.

There have been tense moments on Thursday and Friday as army and police forces have faced off with protesters intent on breaking into electoral offices to burn ballots.