Authorities discovered the colonial-era prison Pademba Road Prison ablaze on Wednesday morning and rushed security officers to secure the scene.
Details about the cause of the violence were sketchy, but it followed the recent confirmation of a coronavirus case in the jail.
Police Superintendant Abu B. Kallon said in a statement on Wednesday that seven had died in the unrest, including two guards and five inmates.
Pademba Road Prison spokesman Cecil Cole Showers told AFP earlier on Wedneday that one guard had died, with several other guards and inmates injured.
Sierra Leone’s government in a statement on Wednesday referred to the unrest as an “attempted prison break”.
It added that it “strongly condemns this attempt to destabilise the security of the state”, saying that an investigation had been launched.
On Monday, the government said that a recently arrived inmate had fallen ill with coronavirus.
The West African state has recorded 116 cases to date, with four fatalities.
The former British colony’s prisons are chronically overcrowded and unsanitary.
Colonial authorities built Pademba Road Prison in 1914 with a capacity for some 300 inmates and the jail now houses over 1,000.
Local NGOs had earlier raised concerns about overcrowding and urged the government to release prisoners with minor convictions to ease the risk of infection.
In the statement on Monday, the government said that it was suspending criminal courts for a month, to stem prison contamination.