Officials say a lot of them were stabbed or slashed; No officers were wounded, and the names of the dead haven't been released.
Jeff Taillon spoke after State Law Enforcement Division agents helped secure Lee Correctional Institution around 3 a.m. Monday.
The S.C. Department of Corrections said on Twitter around the same time that an incident involving "multiple inmate on inmate altercations" in three housing units at the prison began around 7:15 p.m. Sunday.
No officers were hurt but again, 7 inmates have died and 17 others require medical attention.
SC state police have been called to an "ongoing situation" at one of the state's most unsafe prisons.
Camera IconAmbulances outside Lee Correctional Institution in SC, where eight hours of fighting left seven inmates dead and 17 injured.Picture: Supplied, Lee County Fire Dept.
South Carolina Department of Corrections did not immediately return a request for comment Monday morning.
Lee County Fire & Rescue says multiple agencies were called into assist with the "mass-casualty incident" at the prison. One inmate killed another in February.
Lee Correctional Institution, which is reported to have capacity to hold almost 1,800 inmates, is a maximum security facility built in 1993 to house violent offenders and inmates who exhibit behavioral problems.
In March, inmates at the prison held an officer hostage after taking control of a dorm room, The State reported.