At least seven inmates were killed, most of them beheaded, in clashes between rival gangs at a prison in western Guatemala on Wednesday, officials said.
Hundreds of police have been deployed to restore order at the Cantel prison, which houses inmates from both the Mara Salvatrucha gang and their sworn enemies, the Barrio 18.
"Preliminarily, the death of seven inmates is known," the Ministry of the Interior (Interior) said in a statement, describing the situation as a "riot".
The figure was confirmed by the Prosecutor's Office, whose staff managed to gain access to the prison, located 205 kilometers (125 miles) west of the capital.
A spokesman for the National Civil Police (PNC), Jorge Aguilar, earlier told AFP that the violence was caused by a confrontation between two gangs, and that at least six prisoners had been beheaded.
Authorities suspect one of the gangs attacked the other in retaliation for an incident that took place several days ago, Aguilar added.
Gang violence and riots are common at the Cantel prison, which has been condemned by human rights groups for its grim conditions.
Built to house five hundred inmates, it is currently home to over four times that number, police said.
Guatemala's feared and violent gangs are known for their extortion rackets, in which they force businesspeople and bus operators to pay them protection money or face execution.
Gangs are responsible for nearly half of the 3,500 violent deaths a year in Guatemala, authorities have said -- one of the highest death rates on earth.