Two lanes of the southbound 14 Freeway are blocked in the Santa Clarita area following a crash involving about nine vehicles, according to the California Highway Patrol.
Traffic was backed up for miles north into the Antelope Valley for the morning commute.
Fire and highway officials say multiple people have been transported to local hospitals for their injuries, though the nature and number of injuries were not immediately clear.
Los Angeles County Fire Supervisor Phil Ulloa said two ambulances were sent to the scene near Soledad Canyon Road about 35 minutes after fire trucks were deployed.
CHP Officer Francisco Villalobos said calls about the accident came in at about 5:19 a.m. All lanes were blocked for about 30 minutes prior to the carpool lane being opened at 6:30 a.m.
Villalobos said traffic is being diverted at Soledad Canyon Road, where drivers can get off the freeway before quickly getting back on. Traffic, however, is backed up from the north to Ward Road.
A SigAlert was issued at 6:09 a.m. Wednesday and was expected to be lifted about an hour later.
[Updated at 9 a.m. PST, Sept. 18: All lanes of the southbound 14 Freeway were reopened at 7:54 a.m., according to the California Highway Patrol.]