The UNP has said that the violence in Welikada prison ended with security forces committing a massacre.
Mangala Samaraweera of the party said that most of the dead were gunned down by police and the army.
"It's a massacre that has to be condemned. It shows the breakdown of the rule of law in the country," he said, according to Reuters.
"[The] information I have is that most of the prisoners were cold-bloodedly gunned down" he added.
Relatives of some of the victims told BBC Sinhala that after the first clashes died down, soldiers removed prisoners from their cells and shot them.
The secretary to the prison ministry has denied the allegations.
