A battle between rival groups at a prison near Monterrey in northern Mexico has left 52 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.
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 Mr Rodriguez confirmed the numbers.
He 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.
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.
It is not clear if either of the two men are among the victims.
Mr Rodriguez said all those killed were male inmates and that five of the injured were in a serious condition. He said no guns had been involved.
"We are experiencing a tragedy stemming from the difficult situation that they are living through at penitentiary facilities," Mr Rodriguez told a news conference.
"We can confirm the deaths of 52 people. The process of identifying victims continues."
Source: bbc news