Clashes between inmates killed 15 people at a jail in Amazonas state in northern Brazil on Sunday, the regional prison authority said.
The clashes broke out around 11:00 am (1500 GMT) during visiting hours at the facility, located some 28 kilometers (17 miles) from state capital Manaus.
"It was a fight between the inmates. There had never been deaths during the visits," Colonel Marcos Vinicius Almeida told a news conference.
Some of the inmates were stabbed with sharpened toothbrushes, while others had been strangled, Almeida said, adding that an investigation has been opened to determine the cause of the fight.
He emphasized that authorities had reacted within minutes to the Sunday violence, preventing a potentially worse result.