Prisoners responded to the raid by lobbing at least six grenades and opening fire with automatic rifles smuggled inside the Pavon prison. They also attacked authorities with homemade bombs and knives.
Prison authorities originally reported that eight prisoners had been shot dead, but investigators later said seven died while an eighth was treated for gunshot wounds.
More than 3,000 police and the military were deployed in the raid, dubbed operation "High Impact," according to the Guatemalan daily El Periódico. Backed by several military tanks, officers needed hours to take control of the facility, which they say has been run for years by the convicted murderers, kidnappers, street-gang members and drug and weapons smugglers inside.
"We ran into strong resistance. They had grenades, assault rifles, homemade weapons and Molotov cocktails," Guatemala's national director of prisons, Alejandro Giammatei, said in an interview. Giammatei said authorities weren't sure how many inmates the facility housed at the time of the raid.
"I'm sure there are mass graves in the prison where some prisoners may be buried for crimes they committed in prison," Giammatei told the local newspaper Prensa Libre.
Prisoners were transferred to a nearby penitentiary while police and soldiers searched for remaining explosives. In the hours after the raid, they had already recovered 150 firearms.
Located in Fraijanes, just east of the capital, Guatemala City, Pavon is a maximum-security prison and work farm. Small-scale raids in years past have turned up guns, drugs and other contraband, including cellular phones, inside the facility — but authorities had never met such fierce resistance as they did Monday.
Interior Secretary Carlos Vielmann said Pavon had become a den of drugs and organized crime.
"This is a Pandora's box that is only beginning to be opened," Giammatei said.
Vielmann said at least one of those killed while exchanging fire with authorities was armed with an M-16 rifle.
Pavon is home to many prisoners sentenced to more than 50 years behind bars. Some of the prison's most powerful inmates built separate living spaces that allowed them to live in relative luxury away from the rest of the prison.
Authorities declared a heightened state of alert in Fraijanes for eight days to confront a possible violent response to the raid from inmates' allies outside the prison.
Among other things, the security measure expanded authorities' rights to search any vehicle deemed suspicious.