An AFP journalist saw one body at the scene of the shootout, the group's headquarters in the village of Xolapa, Guerrero state , and another seven lying in wake at the homes of relatives.
Ramon Navarrete, president of the Guerrero state Human Rights Commission, said his agency had seen 10 bodies and that it was verifying reports of three more dead.
"We're waiting to get a count of all the dead," Navarrete told reporters.
A state government official, speaking on condition of anonymity, said the death toll could rise to 16 in Xolapa, a village of some 500 people north of the Pacific resort of Acapulco.
The clash did not appear linked to today's congressional, gubernatorial and municipal elections.
The state government said in a statement that the shootout involved rival factions of the United Front for Security and Development in Guerrero State (FUSDEG).
The prosecutor's office is investigating how many people were killed or wounded, the statement said, adding that the group "presumably has a dispute over territory in the Acapulco-Chilpancingo corridor."
Self-defense forces are legal in Guerrero's indigenous communities, where they are allowed to apply their customs for law and order.
The FUSDEG is one of the newest vigilante forces in Guerrero. It was among a slew of new groups that emerged two years ago to combat murders, extortion and kidnappings in the region.
Saturday's shootout took place despite a special federal police and military deployment to protect the elections, especially in Guerrero and neighboring Oaxaca.
In both states, as well as Chiapas, radical teachers have ransacked offices of political parties, burned ballots and clashed with police while vowing to block today's vote.
While protests are the main concern in Oaxaca, Guerrero is one of the country's most violent states, with several drug gangs battling for territory and self-defense forces protecting their communities.
In May, a mayoral candidate was murdered in the town of Chilapa, where two gangs are fighting for control of drug routes.
A woman eyeing the mayor's office in Ahuacuotzingo was killed in March.
Former Acapulco mayor Luis Walton, who is running for governor, said several armed men pointed their guns at his campaign convoy in April and barred him from entering Chilapa.
Elsewhere on Saturday, at least seven supporters of the ruling Institutional Revolutionary Party were wounded when "community police", a legal self-defense force, fired at them in the Guerrero town of Xochistlahuaca, authorities said.
In Tixtla, also in Guerrero, gunmen stopped two election officials and stole some 6,600 ballots from them.
Tixtla is home to the teacher training college of 43 students who were abducted by police and allegedly slaughtered by a drug gang last year. Their relatives also want to block the vote.
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