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.
The clash did not appear linked to today's congressional, gubernatorial and municipal elections.
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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.
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.