At least five people were killed and seven wounded when gunmen opened fire on a group of community defence activists gathered in a plaza in the Mexican state of Michoacan.
About 300 masked community vigilantes wearing similar T-shirts arrived at the city hall in Los Reyes and announced they would take over policing the town, according to Michoacan state prosecutors spokesman Alejandro Arellano.
Minutes later three assailants opened fired on the crowd, killing three members of the self-defence group, a police officer and a passer-by, Mr Arellano said.
Wearing T-shirts that read "For a free Los Reyes" on the front and "Auto-defence group" on the back, the vigilantes had held a news conference to say they would clear the town of criminals and asked to talk to the mayor.
They accused local police of having links to organised crime and also asked for federal police and soldiers to be sent to the town.
Self-defence squads have been forming in recent months in Michoacan and neighbouring Guerrero state by people who say they are fighting violence, kidnappings and extortions carried out by drug cartels.
Concerns have surfaced that the vigilantes might be violating the law, abusing the rights of people they detain, or even co-operating with criminals in some cases.
In Michoacan, the vigilante groups appeared in February and clashes between them and the Knights Templar drug cartel have been intensifying.