At least eight inmates were killed in a prison uprising and more than 50 injured after clashes with guards.
Police said most of the victims in Mahara prison, on the outskirts of Colombo, Sri Lanka’s capital, appear to have been shot.
Prisoners reportedly tried to take control of the prison and hundreds tried to escape.
Violence erupted as authorities tried to quell a protest on Sunday over increasing coronavirus infections in the country’s overcrowded prisons.
The riot started on Sunday when some inmates protested the transfer of the infected prisoner virus from other prisons to Mahara.
Hundreds of additional police officers were sent to assist the guards and strengthen security in the prison.
Hospital bosses said most of the injured inmates were left in a “critical condition”.
Police spokesman Ajith Rohana said prisoners caused “unrest” that “turned into a prison riot,” AP News reported.
He said they “destroyed most of the properties, including prison offices,” and tried to take control.
The guards opened fire, he added.
Ajith Rohana, a senior police officer, said: “We cannot say for sure, but most of the deaths and injuries seem to be due to gunfire.
Shelton Perera, director of Ragama Hospital where inmates were sent from prison for treatment, said: “A majority of inmates injured in the Mahara prison clashes were in critical condition.”
About 500 relatives of prisoners gathered outside the grounds on Monday.
They called on officials to provide them with information about the inmates and to ensure that they are kept safe in the facility.
Sri Lanka has seen an increase in Covid-19 cases in the last month.
There have been thousands of new infections in the country’s overcrowded prisons.
Lags have been protesting in recent weeks, demanding more tests and new isolation facilities for infected inmates.
The latest data from John Hopkins University in Sri Lanka shows that the country has had 22,988 coronavirus cases and 109 deaths.