Protesters burned ballot boxes in several troubled states of southern Mexico on Sunday, in an attempt to disrupt mid-term elections that are being seen as a test for President Enrique Pena Nieto.
Eighty million people are eligible to select the lower house of Congress, hundreds of mayors and nine governors.
The run-up to the poll has been marked by violence, with drug cartels blamed for the deaths of several candidates.
Mid-term elections usually draw a light turnout, but attention was unusually high this time as a loose coalition of radical teachers' unions and activists vowed to block the vote. They attacked the offices of political parties in Chiapas and Guerrero states, and burned ballots in Oaxaca ahead of the vote.
The teachers' demands include huge wage increases, an end to teacher testing and the safe return of 42 missing students from a radical teachers' college. Those students disappeared in September, and prosecutors say they were killed and incinerated by a drug gang. Only one student's remains were identified by DNA testing.