At least 52 people are dead after a battle between the Zetas drug cartel and rivals at a prison in the northeastern Mexican city of Monterrey, authorities said.
The incident was one of the worst in a series of deadly riots in recent years to rock the country's overpopulated prisons, some of which are largely controlled by cartels.
Fighting broke out before midnight in two areas of the Topo Chico prison between supporters of a gang leader known as "Zeta 27" and another group, Nuevo Leon state Governor Jaime Rodriguez said.
"Topo Chico is a...very old prison. A prison with very difficult security conditions," Mr Rodriguez, who survived twoassassination attempts while opposing drug cartels, said.
Monterrey is Mexico's third most populous metropolitan area and home to many of the country's largest corporations.
A 2014 human rights report faulted Topo Chico for not preventing violent incidents.
The prison has long housed members of the Zetas, known for extreme violence.
One Zetas leader was stabbed to death there in September.
It was not immediately clear how the victims died but Mr Rodriguez said there was no gunfire. Flames licked the night sky after inmates set light to food storage areas.
Milenio TV reported that inmates' relatives who had been within the prison's premises for conjugal visits had seen inmates with burns.
12 people were injured, five seriously, the state government said.
Speaking to local radio, Mr Rodriguez acknowledged the public perception that the Zetas dominated the facility and said the prison system was one of his principal concerns.
"The problem is they have people like my brother living with narcos," said an angry relative of an inmate doing time for robbery, waiting for names of the victims at the prison gates.
The state government released the names of 20 victims.
The names of Zeta 27 and a rival known as El Credo were not among them.
Mr Rodriguez said the fighting had been brought under control at about 1.30am (7.30am Irish time) and ruled out a prison break.
Worried family members at one point forced open the prison gates and threw timber and stones at riot police inside, television images showed.
Mr Rodriguez said no women or children were hurt.