At least 13 are dead after a riot broke out between rival groups at La Pila jail in Mexico's San Luis Potosi state.
A group of inmates used homemade knives and picks to attack their rivals starting at 4:15 a.m. local time, saying they were fed up with being harassed by the other group.
Another 65 prisoners were injured, with 22 of them in serious condition.
Spokesperson Gabriela Gonzalez Chong said the violence was a "fight, not an uprising" against prison authorities.
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Officials will investigate whether prison authorities played a role in the fighting.
State authorities have begun naming the dead and have told families waiting for news that the number of fatalities could rise.
Outbreaks of violence are common in Mexico, where jails are overcrowded, many with inmates from rival drug gangs. Disputes between rival cartels and gangs often continue in Mexican prisons, where fights, riots and escapes are regular occurrences.
The National Human Rights Commission reported in 2012 that drug gang-linked prisoners run the interior of many Mexican prisons. In recent years, the Zeta drug cartel has been particularly active in San Luis Potosi.
Promises were made after the last major incident at a Mexican jail, which left 44 inmates dead, to reform the prison system, but no tangible improvements have been made.