The Mexican government has demanded that protesters in the southern city of Oaxaca lift barricades and end their occupation of parts of the city.
The demand came as hundreds of federal police sent by President Vicente Fox were seen arriving in the city.
Three people including a US journalist died in clashes between masked gunmen and leftist protesters on Friday.
The demonstrators are seeking to oust Governor Ulises Ruiz, whom they accuse of abuse of power.
At least six people have been killed since the unrest began in May when striking teachers and left-wing groups occupied the town centre.
Forced to intervene
An interior ministry statement called for "the immediate handover of the streets, plazas, public buildings and private property".
Earlier Mr Fox ordered federal police to go to Oaxaca during Saturday. It was not clear exactly how many were being sent.
The president had resisted sending forces to the region for five months, for fear of involving them in the violent confrontations.
But the BBC's Duncan Kennedy in Mexico City says the latest violence appears to have left him little choice but to intervene.
Gunfire erupted on Friday apparently after armed men tried to remove a blockade set up by protesters.
A series of prolonged shoot-outs followed but it was not clear who fired first.
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 Will Bradley Roland, a cameraman working with the independent news group Indymedia.
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.
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 him 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.