A protester was shot dead when assailants fired on a march of about 8,000 people calling for the governor's resignation in the southern Mexican state of Oaxaca.
The killing on Thursday came after months of escalating political violence in the historic state capital, Oaxaca city.
The 50-year-old protester, identified as Jose Jimenez, was taken to a hospital with bullet wounds and pronounced dead on arrival, according to his wife Clara Jimenez, who was at the hospital sobbing.
Clara said the bullets appeared to come from a house that the marchers passed. After the shooting protesters set fire to the house, a blaze that firefighters later put out.
Protesters also captured four men who they believe carried out the shooting, said Daniel Rosas, a spokesman for the leftist Oaxaca People's Assembly, which organized the march.
Rosas said the men would be presented to media later. In recent weeks, the protesters have captured several men they accuse of attacks and have later handed them over to federal investigators.
Rosas accused Governor Ulises Ruiz of being behind the shooting. Officials in Ruiz's office denied the charge and condemned the killing.
The assembly is demanding the resignation of Ruiz, whom they accuse of using force to repress dissent and of rigging the 2004 election to win office.
Sustained protests descended on Oaxaca in June after police attacked a demonstration of striking teachers demanding a wage increase.
More than 2,000 protesters have camped out in the city center, building barricades, smashing windows and stealing government vehicles. Armed assailants have shot at a radio station and a newspaper that support the protests, injuring one reporter.
Meanwhile, police are nowhere to be seen in the city center.
On Wednesday, two men and a 12-year-old boy headed to join the protest camps in Oaxaca city were shot dead on a road about 250km from Oaxaca, but it is unclear if that killing was connected to the protests.
The confrontations have driven many tourists out of Oaxaca city, which is popular with international tourists for its cobbled streets, markets and cuisine.
Business groups say the conflict has caused losses of more than US$50 million.
Ruiz is a member of the Institutional Revolutionary Party, or PRI, which ruled Mexico for 71 years until President Vicente Fox's election in 2000.
Late Thursday, the PRI released a statement in support of Ruiz.
"The situation that is surging in the state is very worrying because it has the risk of spreading," the party said. "It is an explosive cocktail."
On Sunday, Ruiz asked the federal government to send in federal paramilitary police to restore order.
Interior Secretary Carlos Abascal said that federal troops would not be sent in as he believed the problem could be resolved through negotiations.