Guatemala ? Gang members staged simultaneous riots in at least seven Guatemalan prisons on Monday, attacking rivals with grenades, guns and knives in coordinated chaos that left 31 inmates dead, officials said.

The riots apparently began with attacks by members of the Mara Salvatrucha gang against rivals in gang MS-18, said Interior Minister Carlos Vielmann.

He said 31 inmates died before the riots were brought under control shortly after noon.

An Associated Press photographer saw 18 bodies, many riddled with bullet wounds, carried from El Hoyon prison, which was specifically built to hold gang members in Escuintla, 30 miles south of the capital. A guard and 61 inmates were injured at El Hoyon, and tattooed gang members bleeding from knife wounds were carried from the prison on stretchers.

Dozens of relatives, many of them the mothers of young gang members wept hysterically as stretchers were carried from the prison. The dead were taken to a morgue. So many were injured that they overflowed the capacity of the two local hospitals, forcing officials to take some elsewhere.

Escuintla Gov. Luis Munoz said the riot began with the explosion of two grenades.

The explosions stopped within an hour. Police first began removing the injured, then the dead.