The maximum-security facility that houses about 1,500 inmates in Bishopville, about 55 miles (90 km) from Columbia, is home to some of South Carolina's most violent, longest-serving offenders.
Prisons spokesman Jeff Taillon said State Law Enforcement Division agents were required to help secure Lee Correctional Institution at 3am local time on Monday (8am in the UK).
"The incident at Lee CI resulted in 17 inmates requiring outside medical attention and 7 inmates were killed", the department said on its Twitter account.
South Carolina Department of Corrections did not immediately return a request for comment Monday morning.
Kathua case: BJP demands action against Jammu and Kashmir Cong chief
He evaded questions on why Rahul Gandhi and the Congress remained silent for about three months on the rape and murder case. Similarly, Romi Sharma is also a Youth Congress president and vocal supporter of the manch.
Lee County Fire/Rescue said ambulances from multiple jurisdictions lined up outside to tend to the wounded.
These deaths at Lee are the most in any SC prison in recent years.
Two officers were stabbed in a 2015 fight by two inmates that left one of them dead.