At least eight prisoners were killed and seven others injured in a riot between prisoners in Mexico's Pacific coast state of Colima.
The fight broke out on Tuesday between two rival groups of inmates at the Center for Social Reintegration, local media quoted police as saying.
Security forces retook control of the cell block about an hour later, and the situation is under control.
In a statement, the provincial governor's office said an investigation has been launched into the incident. It also offered condolences to the families of the prisoners who lost their dear ones.
Inmates are said to have control over Mexico's poorly administered prisons, leading to problems.