Among the dead are six prison guards, but most of the victims are inmates.
According to local police, the violent clashes broke out on Monday night between inmates from two rival drug gangs.
"There are victims with gunshot wounds. Some have cuts, some are badly bruised," says hospital director Pushpa Gamlath.
During the violence, female inmates climbed onto a roof and demanded to be released. Parts of the roof collapsed and several of the women were injured.
Justice Minister Harshana Nanayakkara says he is “deeply shocked and saddened.” It is the deadliest prison riot in Sri Lanka in years, authorities say.
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