Guatemalan prison officials say the number of people killed in a fight between inmates has risen to 16.
Police have not yet been able to retake control of the prison in the town of Escuintla, about 60km south of the capital, Guatemala City.
Officials said they would wait for reinforcements before entering the jail as the security forces were currently outnumbered four-to-one by the rioters. The jail is notorious for overcrowding and violence, BBC reported.
More than 3,000 prisoners are held at the jail, which was built to house just 600. About 2,300 inmates are currently engaged in a stand-off with 500 members of the security forces, Deputy Interior Minister Elmer Sosa said. The minister confirmed that some of the inmates had guns.
Sosa said that there were conflicting theories as to what had triggered the violence.
He said there were reports that convicted gang members had clashed with inmates without any gang affiliation after visiting hours on Sunday.
