Officials said the inmates at Lima's Lurigancho jail were fighting over trafficking in food and alcohol and had made no demands to authorities.
"It's a conflict between ... internal mafias," said Wilfredo Pedraza, head of Peru's National Prison Institute.
"Unfortunately in Lurigancho ... guns get in because of negligence and corruption. It's a reality."
Pedraza said prison authorities had conducted a search of the jail that houses 8000 inmates in late December and early January after similar arguments between inmates but had failed to find guns.
About 50 weapons were found in the jail in 2003 and 44 in 2004.