At least 20 inmates have been killed in a crackdown by Myanmar security forces against Friday's protest at Yangon's Insein prison, the Democratic Voice of Burma reported.
According to the local media outlet, which cites an unnamed source close to the military, the soldiers opened fire on protesters. Five women were among those killed.
The military also arrested staff at the prison, where hundreds of political prisoners are held, the outlet reported on Saturday night.
The protest began on Friday morning in the women's annex to demand medical care amid the rapid spread of Covid-19 inside the prison, the Association for the Assistance of Political Prisoners (AAPP) said.