At least 55 prisoners have been found dead in four jails in the Brazilian city of Manaus after clashes between rival gangs, authorities said.
Forty prisoners were found strangled on Monday while a further 15 inmates died from choking and stabbing on Sunday.
Prison clashes often spread rapidly in Brazil, where drug gangs have de facto control over nearly all jails.
Family members protested by blocking roads and clashing with police outside the prisons.
Local Brazilian authorities said jails are very overcrowded with more than 700,000 people in prison at the moment.
Violence isn't uncommon in Brazilian's prisons. In January 2017, nearly 150 prisoners died during three weeks of violence, as large drug factions butchered one another.
Brazil's new far-right president, Jair Bolsonaro, has vowed to regain control of prisons and build more of them to ease overcrowding.
A federal task force is being sent to Manaus in an effort to halt the violence.