Fifty-two inmates were killed and 12 injured in a brutal fight between two rival factions at a prison in northern Mexico on Thursday, the state governor said.
Nuevo Leon Gov. Jaime Rodriguez told a news conference the fight involved a faction led by a member of the infamous Zetas drug cartel.
Rescue workers could be seen bringing injured inmates from the Topo Chico prison in Monterrey, at least some with burns.
The riot 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.
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.
The fire appeared to have been extinguished by shortly after sunrise.
Witnesses said the fire broke out at about 12:30 a.m. 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.
A 2014 human rights report said the Topo Chico prison was packed with 25 percent more criminals than it could hold, and faulted it for not preventing violent incidents. The prison has long housed members of the vicious Zetas drug gang. One Zetas leader was stabbed to death there in September.
In 2012, at least 44 inmates died in another Nuevo Leon prison when members of the Zetas plotted with prison guards to stage an elaborate escape.
In 2013, at least 13 people were killed and 65 injured in a prison riot, which was blamed on gang violence, in the central Mexican state of San Luis Potosi.
Thursday's riot was a harsh blow to Nuevo Leon, where many were uplifted when Jaime Rodriguez, a blunt, outspoken rancher with a penchant for cowboy hats known as "El Bronco," or "the gruff one," defeated President Enrique Pena Nieto´s ruling party last year to win the governorship.
Rodriguez, a former member of Pena Nieto's Institutional Revolutionary Party (PRI), waged a campaign that capitalized on widespread disaffection with the established parties. He was the first independent candidate to win such a post in modern Mexico.