Eight big rigs were involved in a collision on the 60 Freeway in Chino Valley on Tuesday afternoon, officials said.
The "chain-reaction" crash happened on the eastbound side of the highway near Central Avenue, leaving just one lane open to traffic, the Chino Valley Fire District tweeted at 12:19 p.m.
Four patients were treated at the scene, one of them transported to the hospital with minor injuries, the agency said.
Only the carpool lane was accessible as of 3:35 p.m., with the remaining lanes expected to be closed for another hour to an hour and a half, according to the California Highway Patrol.
Officials did not shut down westbound lanes but traffic there was heavy.
Authorities provided no further information.