At least 11 people have been killed in clashes between rival vigilante groups in Michoacan state, western Mexico.
The two groups confronted each other in the town of La Ruana.
The vigilante groups were created almost two years ago by locals who said the security forces had not done enough to protect them from drug cartels.
Earlier this year, the government tried to gain control of the vigilantes by integrating them into a rural police force and registering their weapons.
Michoacan Security Commissioner Alfredo Castillo said the clashes were triggered by a "historic rivalry" between their leaders.
During Tuesday's two-hour shoot-out, five members of a group led by Hipolito Mora and six followers of his rival, Luis Antonio Torres, were killed.
Mr Mora's son Manuel was among those shot dead, officials said.
The "self-defence groups" were set up in February 2013 to fight a drugs cartel calling itself the Knights Templar.
Earlier this year, they took control of a number of towns in the western state and drove out the Knights Templar, who had been extorting money from local businessmen and farmers.
But tensions soon emerged between the different groups, which accused each other of having been infiltrated by the very same drugs gang they were set up to fight.
Fights over land
There were also disputes as the groups seized ranches and land previously held by the Knights Templar.
In March, Mr Mora was arrested on suspicion of killing two vigilantes from another group.
He was freed two months later, after prosecutors determined there was no evidence linking him to the crime.
On Tuesday night, the government said it had sent federal police officers to La Ruana to get the vigilantes under control.
The shoot-out comes at a time when tens of thousands of Mexicans have been taking part in demonstrations demanding the government do more to tackle the links between drug cartels and local law enforcement.
The high levels of collusion between the two were highlighted by the case of 43 students who are believed to have been killed by a drugs gang in neighbouring Guerrero state.
The 43 disappeared from the town of Iguala after having been handed over by the municipal police force to members of a local drugs gang.
Members of the gang say they killed them and burned their bodies, but so far only the remains of one of the 43 students have been identified.