At least 52 men died in a northern Mexican prison on Thursday as inmates ignited a fire during a brawl between two rival groups, authorities said.
Governor Jaime Rodriguez said the clash erupted at the Topo Chico prison in the industrial city of Monterrey before midnight on Wednesday and that authorities brought it under control at 1:30 am on Thursday.
"We are experiencing a tragedy stemming from the difficult situation that they are living through at penitentiary facilities," Rodriguez told a news conference.
"We can confirm the deaths of 52 people. ... The process of identifying victims continues," he said, without specifying whether prison guards were among the dead.
He said all the victims were men, rejecting fears that women or children may have been inside at the time of the riot.
Twelve inmates were injured, five of them in serious condition.
The fight involved a group led by a leader of the Zetas drug cartel, he said. During the brawl, inmates set fires in supply rooms.
Troops and federal police were deployed inside the prison to keep it under control. Rodriguez said no inmates escaped from the prison.
Ambulances were sent to the prison while dozens of relatives desperate for news flocked to entrance, throwing rocks and pulling the gate open as riot police blocked their way.
Other relatives shouted through a fence, hoping to hear information from the inmates.
TV footage showed a fire inside the facility in the middle of the night.
Some relatives of prisoners formed a line by holding hands to block a boulevard.
"We will stay here blocking this avenue until they give us an answer. We want to know how our relatives are doing because they are telling us that there are more than 50 dead and no authority is giving us answers," Ernestina Grimaldo, whose son is a prisoner, told AFP.
The riot erupted on the eve of Pope Francis' trip to Mexico, during which he is due to visit another notorious prison, in the northern border city of Ciudad Juarez.
Mexican penitentiaries are notoriously overcrowded and massive prison breaks have taken place in recent years.
In February 2012, 44 inmates were killed and another 30 escaped from another Monterrey prison, known as Apodaca.
Even the country's top maximum-security prison, the Altiplano near Mexico City, weaknesses were exposed when drug kingpin Joaquin "El Chapo" Guzman escaped through a tunnel in July. He was recaptured in January.
Ruth Villanueva, an expert at the governmental National Human Rights Commission, told local media last year that there was a serious crisis at the country's prisons, with 72 of them overcrowded by more than 20 percent.
President Enrique Pena Nieto's administration vowed to reform the penitentiary system following Guzman's escape last year.