Among the dead were said to be guards after clashes at the prison in Negombo, about 22 miles from the country’s biggest city, Colombo.
It is unclear exactly what caused the violence, which began over the weekend and continued until Monday.
Officials are still trying to account for the dead and injured.
Prison spokesman AC Gajanayake said the violence started among inmates and when prison guards attempted to intervene, “they (inmates) started attacking the prison officials”.
Army spokesman Brigadier Waruna Gamage said the “military has been requested to provide support to the police but at the moment they are on standby”.
A local TV station showed images of police deployed outside the gate of the prison as a police bus carried injured inmates.
Read more from Sky News:
Iranians attend funeral procession for supreme leader
Sky to pay £1.6bn for ITV’s broadcast and streaming division
Victims include both inmates and prison officials, the channel reported.
Sri Lankan prisons are highly congested, with more than 39,000 inmates crowded into a system with a total capacity of just 10,000.
