Rival Guatemalan gang members battled each other with grenades, guns and knives at prisons throughout the country on Monday, leaving at least 30 people dead. About 80 inmates were injured in the fighting between the Mara Salvatrucha and Mara 18 street gangs, the government said.
Survivors said the breakdown of a peace pact between the two gangs in the prisons may have caused the trouble.
Hundreds of police and soldiers armed with automatic weapons and firing tear gas needed several hours to regain control of the prisons.
Witnesses saw emergency services workers carry the bodies of 18 prisoners out of one of the prisons, dubbed "The Hole" by inmates, in the town of Escuintla in southern Guatemala. Corpses were tattooed with gang symbols and some were mutilated.
Hundreds of weeping relatives of the dead surrounded the town morgue, trying to identify family members.
Many of the dead were killed by hand grenade blasts and suffered knife and gunshot wounds, the government said.
Elsewhere, eight people were killed in the Pavon correctional facility near Guatemala City and four more killed in two other prisons, Francisco de la Pena, head of Guatemala's prison service, said at a news conference.
"We believe they were armed with grenades, pistols and mini-Uzis," Hugo Martinez of the government human rights commission Copredeh said after entering the Escuintla jail.
Martinez said there had been no complaints of police brutality during the Escuintla riot.
Guatemalan prisons are chronically underfunded and corruption among prison staff means weapons and drugs are widely circulated.
Street gangs known as "maras" are widespread in Central America and are known for their use of extreme violence and a deadly feud between the two principal gangs.
The groups are especially feared in Honduras, where 23 people were killed in a Christmas-time bus massacre blamed on the gangs last year.
In April, former Honduran prison chief Oscar Sanchez was jailed for 19 years for his role in the killing of 68 inmates, mainly gang members, during a prison riot in 2003. Dozens more gang members died in a Honduran prison fire in 2004.