About 50 people lashed a policeman to a light post and beat him because he was allegedly caught trying to rob a house, the latest in a string of public beatings of suspected thieves in the restive southern state of Oaxaca.
It was unclear whether the crowd released the alleged burglar, Manuel Dominguez, after the beating late Tuesday. Dominguez was identified by his police badge.
"We haven't had time to check what happened to him," local police director Aristeo Lopez said Wednesday.
Authorities have been kicked out of the center of the tourist city 350 kilometers (220 miles) southeast of Mexico City. Police occupy only a few neighborhoods, and have lost control of the city as a whole.
Protesters are demanding the resignation of Gov. Ulises Ruiz, whom they accuse of rigging the 2004 election to win office and of repressing dissent with violence.
The protests began in May when teachers went on strike to demand higher wages. In June, police attacked a demonstration and the teachers took up the call for Ruiz to step down.
On Wednesday, the teachers were to release the results of a vote on whether to return to classes. Previous votes have failed, and the strike has kept schools closed for months.
Some teachers have defied the union and tried to open some schools. But masked protesters have been blocking children and their parents as they try to return to classes.
Also Tuesday, police clashed with protesters in Cosolapa, firing automatic weapons into the air to scatter a crowd protesting Ruiz's visit to the town. No one was injured, but television images showed people ducking for cover behind buildings as police fired repeatedly.
In an interview with the Associated Press, Ruiz said the protesters had thrown firecrackers at the police, forcing them to respond.
Despite public threats against him, Ruiz said he knows of no direct threats against his life. He has refused to step down.
"I'm not going to cancel my visits. I take care not to hurt other people but there are a lot of people who want to see me," Ruiz said. "I have killed nobody. My hands are clean."
Protest leader Florentino Lopez promised to increase actions against the governor after the shooting, saying federal roads would be blocked.
Five people have been killed by police or armed groups, including a teacher who was hacked to death by ice picks. The unrest has scared away tourists, costing the city more than US$300 million (euro238 million), according to local business associations.
Ruiz repeatedly has called on the federal government to send in the army to restore order. But President Vicente Fox's administration has so far refused, saying it prefers to negotiate.
On Tuesday, federal Attorney General Daniel Cabeza de Vaca said he has arrest warrants for 10 protesters for damaging federal property.
A group of 50 teachers went to the United Nations office in Mexico City on Tuesday to demand the international body intervene and ask Ruiz to step down.
UNICEF has called on all parties to resolve the conflict and allow children to return to school.