At least 13 people are dead after a battle broke out between prisoners at a jail in the central Mexican state of San Luis Potosi.
A group of inmates used homemade knives and picks to attack rivals at the La Pila prison, the state attorney general's office was quoted as saying.
Authorities took several hours to bring the fighting under control.
Deadly outbreaks of violence are common in Mexico's overcrowded jails, which house inmates from rival drug gangs.
Dozens of people were injured - some seriously - in the fighting that broke out at La Pila, situated in the state capital, early on Saturday morning, officials said.
The authorities in the northern state of San Luis Potosi have begun to name the dead and warned concerned families waiting for news that the number of fatalities may rise.
Violence began when a group of prisoners took action after being harassed by other inmates, news agency AP quoted the state attorney general's office as saying.
Rivalries between criminal gangs frequently spills over into Mexico's antiquated and dangerous prisons, correspondents say. Studies say some prisons are effectively run by gangs.
Human rights groups say the penal system suffers from chronic overcrowding and is in urgent need of an overhaul.
Despite assertions that the prison system would be reformed after the last major incident, in which 44 inmates were killed, there have been no tangible improvements, neither during the final year of the previous administration nor in the first six months of President Enrique Pena Nieto's government, says the BBC's Will Grant in Mexico City.
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