Six people were transported to Riverside County Medical Center, the CHP said. At least three people suffered minor injuries. No serious injuries were reported.
A cause of the accident was not immediately known.
Sofia Copado was on the freeway when the accident happened.
"We started seeing smoke, everybody slowed down to a dead stop, and we saw smoke coming up. We started seeing flames and people were getting out of their cars," Copado told Eyewitness News in a phone interview.
Traffic was diverted off at Redlands Boulevard. After about three hours, all lanes were reopened by 8 p.m.
The CHP first reported the closure affected Riverside. Information from fire officials clarified the location as Perris.