Police fired in the air to scatter a crowd of leftist demonstrators in the southern Mexican state that has been paralyzed by months of protests to oust the governor.
TV images on Tuesday showed the protesters in the town of Cosolapa, about 200km north of the state capital Oaxaca, ducking for cover behind buildings as police fired repeatedly with automatic rifles.
There were no immediate reports of injuries.
The leftists were protesting a visit to Cosolapa by Oaxaca Governor Ulises Ruiz, who they accuse of rigging the 2004 election to win office and repressing dissent with violence.
In an interview, Ruiz said the protesters had thrown firecrackers at the police, forcing them to respond.
The governor said he has had life threatened various times recently but that won't stop him touring his state.
"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.
The protests against Ruiz began in May when teachers went on strike to demand higher wages.
In June, police attacked one of their demonstrations in the colonial capital, 350km southeast of Mexican City, and the teachers added to their demands the resignation of Ruiz.
Soon they were joined by leftists, students and Indian groups and protesters blocked streets in the center of Oaxaca, burning buses and taking over radio stations.
Five people have been killed by police or armed groups. The unrest has scared away tourists, costing the city more than US$300 million, according to local business associations.
The unrest has created a state of lawlessness in Oaxaca, leading to daily episodes of vigilante justice.
On Tuesday, a group of about 50 people in a working class Oaxaca neighborhood beat, stripped and tied up a local police officer who allegedly tried to burgle a house. Other police officers were nowhere to be seen.
On Sunday, a majority of union delegates voted to end the strike, which is affecting 1.3 million children across Oaxaca state. But the vote was quickly annulled by those who support continuing the strike. Teachers now plan to meet to hold another vote sometime this week.
Some teachers have defied the union by trying to open some schools, but protesters have been blocking children and their parents as they try to return to classes.
Last week, the federal Senate voted that there were no grounds to remove Ruiz from office.
Ruiz has repeatedly 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 continue negotiating.
On Tuesday, Federal Attorney General Daniel Cabeza de Vaca said he has arrest warrants for ten protesters for damaging federal property.
Also a group of 50 teachers went to the UN office in Mexico City to demand the international body intervene and ask Ruiz to step down. Ruiz has refused to resign.
UNICEF has called on all parties to resolve the conflict and allow children to return to school.