Authorities said the bus lost control at around 10:30 a.m. Sunday on Interstate 84, which was covered in ice and snow.
The bus crashed through a guardrail and went down an embankment a few hundred feet.
Emergency crews were using ropes to help retrieve people from the crash scene. Police say the charter bus was carrying about 40 people. Authorities have not released information on the company that owns the bus.
The Associated Press contributed to this report.