The Oregon Department of Corrections said a series of fights broke out about 5:30 p.m. Monday in the dining room, leading to a lockdown in part of the Eastern Oregon Correctional Institution.
After-dinner fighting led to a complete lockdown.
The department said an investigation was underway as to the cause of the fighting. In a statement Tuesday, officials said the prison was running on "modified operations," and visits from outside were cancelled. One inmate had to leave the grounds for treatment but was returned on Tuesday.
The prison is a medium-security prison with 1,700 men in custody.