A battle between rival groups at a prison near Monterrey in northern Mexico has left 49 inmates dead.
Nuevo Leon state Governor Jaime Rodriguez said 12 other people were injured in Topo Chico jail after prisoners fought with "sharp weapons, bats and sticks".
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.
Some threw sticks and rocks and tried to pull the prison gate open as riot police blocked their way.
"They haven't told us anything," said the mother of one inmate, who gave her name only as Ernestina.
"They said that until there is order they won't let us in. Everything is in disorder, and nobody is telling us anything."
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.
Mr Rodriguez had earlier put the number of inmates killed at 52 before revising the figure down.
The reason was not made clear, but several inmates are registered more than once at the prison, with different names.
Forty of the 49 prisoners have already been identified.
Mr Rodriguez said the fight had started around midnight and lasted 30 to 40 minutes, during which time the two groups of inmates set fire to a storage area.
He said one faction was led by a member of the notorious Zetas drug cartel, Juan Pedro Zaldivar Farias, also known as Z-27.
Mr Rodriguez said the other group was led by Jorge Ivan Hernandez Cantu, whom Mexican media identified as a member of the rival Gulf cartel.
The faction leaders are not among the 40 bodies identified so far.
Mr 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," Mr Rodriguez told a news conference.
Speaking later, he 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.
Topo Chico prison
- Heavily overcrowded, with prisoners sleeping in the corridors
- Cells have no water, light or ventilation, according to the UN Special Rapporteur on Torture
- Female prisoners complain of being forced into sexual slavery by male inmates
- Members of rival drug gangs Los Zetas and the Gulf cartel target each other
- In 2011, Topo Chico's security chief was abducted, killed and his body dismembered
- Previous escape attempts have involved digging tunnels and even a grenade attack from outside the prison walls
Earlier, state public security spokesman Antonio Arguello told AFP news agency that authorities had quickly formed a security cordon around the facility to ensure no inmates escaped.
Witnesses reported hearing shouts and explosions when the riot began. Flames and smoke were then seen coming from the prison.
Gang violence and break-outs are common in Mexico's notoriously overcrowded and corrupt prison system.
In 2013, 13 people died in a battle between inmates at the La Pila jail in the central state of San Luis Potosi.
A year earlier, 44 inmates were killed in a riot at the Apodaca prison in Monterrey. Another 30 prisoners escaped. The prison's director was later sacked.