Police in Mexico say at least 14 inmates have been killed in a prison riot in Matamoros, in Tamaulipas state, close to the US border.
Local media say rival gangs fighting for control of the prison attacked each other with a variety of sharp weapons.
Soldiers and police eventually regained control of the penitentiary.
The prison system is struggling to cope with the large number of drug offenders jailed in President Felipe Calderon's battle against powerful drug cartels.
Many prisons are overcrowded and allegations of corruption are widespread.
Last month, prosecutors accused prison officials at a jail in Gomez Palacio in Durango state of allowing inmates to leave the prison at night to carry out revenge killings, even providing them with the weapons to carry out the attacks.
Police spokesman Hector Walle told Reuters news agency the inmates in Matamoros used homemade weapons in the fight, which quickly spiralled out of control.
Matamoros, just across the border from Brownsville, Texas, has seen an increase in violence as rival drug cartels fight for control of the smuggling routes into the United States.
The region is at the centre of a violent battle for territory between the Gulf cartel and the Zetas, who used to work as hitmen for the Gulf cartel but have since become their rivals.
The Mexican Centre for Research and National Security (Cisen), says more than 28,000 people have been killed in drug-related violence since December 2006.
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