Hospitals say they are struggling to cope with the number of injured

Hundreds of armed police and soldiers were deployed and fired tear gas to regain control of the prisons.

At least 18 of the inmates died during a fight at a prison known as "The Hole" in a town south of Guatemala City.

The gangs emerged in the US and later spread to Central America when their members were deported back home.

'Ominous warning'

The latest incidents are thought to have erupted after members of one of Central America's most notorious gangs, the Mara Salvatrucha, clashed with members of their main rival, Mara 18.

The gangs appear to have co-ordinated the fighting using mobile telephones, said Guatemala's Interior Minister Carlos Vielmann.

Officials said an array of weaponry - including knives, automatic rifles and hand grenades - had been smuggled into the prisons.

Some of the inmates were killed by gunshot wounds and hand grenade blasts - others were found with their throats cut.

The bloodiest fighting appears to have taken place in the "The Hole" prison in Escuintla town.

Witnesses saw corpses covered in the trademark tattoos of their gangs. Some bodies were said to have been mutilated.

Eight men were also killed in the Pavon correctional facility near Guatemala City, said the head of the country's prison service.

The BBC's Paul Keller, says the rioting is an ominous warning to the authorities in Honduras and El Salvador as well as Guatemala.