Commuters are slowly crossing the Dumbarton Bridge after a major-injury crash brought westbound traffic to a halt for nearly two hours, the California Highway Patrol said.

At least four vehicles were involved in a collision reported about 6:30 a.m. at the midspan of the bridge, the CHP said. All westbound lanes were blocked. Details, including the number of injuries and their severity, were not immediately available.

Tow trucks and emergency personnel were summoned to clear wrecked vehicles and tend to the injured. Eastbound traffic was also narrowed for a time to give them access to the crash site.

Westbound traffic was opened around 8:15 a.m., but the residual backup is heavy and the CHP advises motorists to seek alternate routes if possible.