Three people, including a US journalist, have been killed in the Mexican city of Oaxaca.
President Fox has vowed to end the conflict before he leaves office
It is unclear who is responsible for the shootings which officials say began after locals clashed with protesters.
At least six people have been killed in the conflict that began in May when striking teachers and left-wing groups occupied the town centre.
Protesters accuse state Governor Ulises Ruiz of abuse of power, and say they will not back down until he is ousted.
Gunfire erupted when armed men tried to remove a blockade set up by protesters, the Associated Press news agency quoted a police official as saying. Both sides fired but it was not clear who shot first, he said.
The Oaxaca People's Popular Assembly, which is leading the protests, accused off-duty local policemen of carrying out the shootings.
The dead journalist has been named as Bradley Roland Will, a cameraman working with the independent news group, Indymedia.
'No peaceful way out'
Thousands of schools have been closed since the strike began in May, leaving 1.3 million children out of school.
The teachers staged the walk-out, demanding higher pay and better working conditions.
Protesters want Governor Ruiz removed from office
After police attacked one of their demonstrations in June, they extended their demands to include a call for the resignation of Gov Ruiz. The teachers were joined in their protest by left-wing groups.
This week, striking teachers voted to return to classes but many protesters say they will not back down until Gov Ruiz is removed from office.
Critics accuse Mr Ruiz of corruption and repressive tactics against dissenters, whose roadblocks have driven tourism from the city and hurt business.
Last week, Mexico's Senate decided by a 74-31 vote that the state government had not ceased to function, a condition necessary to remove a governor from office.
But the Senate recognised that conditions of "ungovernability" existed in the state and criticised Mr Ruiz for failing to bring months of violent protests to an end.
President Vicente Fox has vowed to end the conflict before he leaves office on 1 December but negotiations to find a peaceful way out have so far failed.