The head of security for Amazonas state, Sergio Fontes, told a news conference that the death toll could rise as authorities get a clearer idea of the scale of the rebellion sparked by a fight between rival drug gangs.
Fontes told reporters that several of the dead had their decapitated bodies thrown over the prison wall - and that most of those killed came from one gang.
"This was another chapter in the silent and ruthless war of drug trafficking," he said.
Pedro Florencio, the Amazonas state prison secretary, said that the massacre was a "revenge killing" that formed part of an ongoing feud between criminal gangs in Brazil.
The riot began late Sunday and was brought under control later today, Fontes said.
Authorities were still counting the prisoners to determine how many had escaped, he added, with reports that up to 300 fled.
Just as the riot began in one unit of the Anisio Jobim prison complex, dozens of prisoners in the second unit started a mass escape in what authorities said was a coordinated effort to distract guards.
Overcrowding is extremely common in Brazil's prisons, which suffer endemic violence and what rights groups call medieval conditions with cells so crowded prisoners have no space to lie down and food is scarce.
The Anisio Jobim prison complex currently houses 2,230 inmates despite having a capacity of only 590.