A brutal fight between rival factions has killed 52 inmates and injured 12 at a prison in northern Mexico, the state governor said.
Nuevo Leon governor Jaime Rodriguez said there were no reported escapes and the battle, which sent flames billowing into the pre-dawn sky, did not involve guns.
Mexico's deadliest prison riot in many years broke out just six days before Pope Francis is scheduled to visit another Mexican prison, in the border city of Ciudad Juarez in Chihuahua state.
The fight involved a faction led by a member of the infamous Zetas drug cartel, Juan Pedro Zaldivar Farias, also known as Z-27, Mr Rodriguez said.
The Zetas once nearly controlled the area around Monterrey and Zaldivar Farias was a suspect over the 2010 shooting death of American David Hartley on Texas's Falcon Lake. Hartley was reportedly shot while touring the lake with his wife on jet-skis.
The governor said the fight broke out between supporters of Zaldivar Farias and Jorge Ivan Hernandez Cantu, who has been identified by local news media as a member of the rival Gulf drug cartel.
The war between the two cartels bloodied Nuevo Leon state and neighbouring Tamaulipas state between 2010 and 2012.
Images broadcast by Milenio Television showed flames leaping from the prison, with a crowd of people bundled against the cold gathered outside the prison. Some shook and kicked at the prison gates, demanding to be allowed in. Rescue workers could be seen bringing injured inmates away from the Topo Chico prison in Monterrey, at least some with burns.
Mr Rodriguez said the fight broke out just before midnight, and that inmates set fire to a storage area, causing a thick cloud of smoke.
The deadliest prison riot in recent memory also occurred in Nuevo Leon, in February 2012, when members of the Zetas murdered 44 members of the Gulf cartel at the overcrowded Apodaca federal prison. One month earlier, 31 inmates died in a brawl in the neighbouring state of Tamaulipas. That riot involved inmates fighting with makeshift knives, clubs and stones.
Mexico's official National Human Rights Commission reported in 2013 that the country's prison system is plagued by violence and cases of inmate control, symptoms of corruption and lack of resources.
The report, based on visits and interviews at 101 of Mexico's most populated prisons, found that 65 of the facilities were run by inmates, not authorities.