Fifty two inmates have been killed in a battle between rival groups at Topo Chico jail near Monterrey, northern Mexico.
Other 12 people were injured in Topo Chico jail after prisoners fought with “sharp weapons, bats and sticks”, Nuevo Leon state Governor Jaime Rodriguez said.
A fire was also started in a storage room. Officials say the situation is under control and no inmates escaped.
Crowds of relatives outside the jail blocked roads, demanding information.
The incident comes just days before Pope Francis is due to visit a prison in the northern city of Ciudad Juarez, an area notorious for violence between drugs cartels.
Initial reports said up to 60 people had died and anxious families of inmates had to wait several hours before Jaime Rodriguez confirmed the numbers.
The governor said the fight had started around midnight and lasted about 30-40 minutes, during which time the two groups of inmates set fire to a storage area.
Jaime Rodriguez said one faction was led by a member of the notorious Zetas drug cartel, Juan Pedro Zaldivar Farias, also known as Z-27.
He said the other group was led by Jorge Ivan Hernandez Cantu, whom Mexican media identified as a member of the rival Gulf cartel.
It is not clear if either of the two men are among the victims.
Jaime Rodriguez said all those killed were male inmates and that five of the injured were in a serious condition.
“We are experiencing a tragedy stemming from the difficult situation that they are living through at penitentiary facilities,” he told a news conference.
“We can confirm the deaths of 52 people. The process of identifying victims continues.”
Speaking later, Jaime Rodriguez said security was being beefed up at other prisons and some inmates had been transferred out of Topo Chico.
He said that although rioters had not had guns, one inmate appeared to have been shot dead by a guard who was protecting a group of women inmates.
A report by the National Human Rights Commission in 2014 said the Topo Chico prison housed about 4,600 inmates but was only designed to hold 3,635.