At least five people were killed and eight others wounded when gunmen opened fire on about 200 members of a self-defense group protesting police corruption in Los Reyes, a city in the western Mexican state of Michoacan, prosecutors said.
Investigators suspect that the gunmen work for a drug trafficking organization, the Michoacan Attorney General's Office said.
The gunmen opened fire on the demonstrators, who were armed with sticks and machetes, around 1:00 p.m. Monday outside city hall, an AG's office spokesman told Efe.
The protesters alleged that municipal police officers have links to the Los Caballeros Templarios drug cartel, the AG's office spokesman said.
The gunmen arrived on the scene in SUVs and opened fire on the demonstrators with assault rifles.
Three members of the community self-defense group and a policewoman from the neighboring city of Periban were pronounced dead at the scene.
The policewoman was on her day off, officials said.
A man hit by gunfire as he left a bank next door to city hall was pronounced dead at the municipal medical center.
Eight wounded members of the vigilante group were taken to the medical center, where two are listed in srious condition, officials said.
Residents of the cities of Coalcoman, Buenavista Tomatlan, Los Reyes, Tepalcapetec and Aquila took up arms on Feb. 24 against Los Caballeros Templarios.
Gunmen suspected of being on the payroll of the Caballeros Templarios, meanwhile, engaged in a shootout with a self-defense group in the town of San Jose de la Montaña, killing a member of the vigilante group, police told Efe.
The Caballeros Templarios, La Familia Michoacana and Jalisco Nueva Generacion drug cartels have been fighting for control of Michoacan.
The gangs have been battling each other, as well as new armed vigilante groups that call themselves "community police forces," in different cities and towns.
The Caballeros Templarios gang was founded in March 2011 by former members of La Familia Michoacana and deals in both synthetic drugs and natural drugs.
The cartel has been fighting rival gangs for control of turf and smuggling routes in Guerrero and Michoacan states, both of which are on the Pacific.
Michoacan's forests and mountains are used by drug traffickers to grow marijuana and produce synthetic drugs.
Los Caballeros Templarios is suspected of carrying out murders, staging kidnappings and running extortion rackets that target business owners and transport companies in Michoacan's 113 municipalities, officials said. EFE