Fifty-two inmates were killed and 12 injured in a brutal fight between two rival factions at a prison in Monterrey, Nuevo León, the state governor said.
Governor Jaime Rodríguez told a news conference the fight at the Topo Chico prison involved a faction led by a member of the Zetas drug cartel.
Rescue workers could be seen bringing injured inmates, at least some with burns. Four of the injured inmates are: Óscar Reynaldo Cázarez González, Axel Lobato Mendoza, Jorge Alberto Aguirre Treviño and Hugo Roberto Escobar Rincón.
The governor added that announcing the names of the dead inmates will take time because the prison has 3,800 convicts.
The riot broke out just six days before Pope Francis is scheduled to visit another Mexican prison, in the border city of Ciudad Juárez in Chihuahua state.
Flames leaped from the prison while 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.
The fire appeared to have been extinguished by shortly after sunrise.
Witnesses said the fire broke out at about 12:30 a.m. (1:30 a.m. EST; 0630 GMT) amid shouts and sounds of explosions. A thick cloud of smoke rose, apparently from inmates burning mattresses.
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.
In one of the worst incidents in 25 years, 44 inmates died in a prison massacre in February 2012 in Apodaca, Nuevo León. After the incident, three top prison officials and 26 guards were accused of helping inmates escape in the confusion. The governor of Nuevo León state says 52 inmates were killed and 12 injured in a brutal fight between two rival factions Thursday at a prison in northern Mexico.