Thousands of dockworkers did not show up for the morning shift, leaving ships and truck drivers idle at ports from Long Beach to Seattle, said Steve Getzug, spokesman for the Pacific Maritime Association, which represents all 29 ports.
About 10,000 workers belonging to The International Longshore and Warehouse Union joined the anti-war protest because they claimed the big shipping companies were profiting from the war.
However, port officials cast doubts over the war as the reason for the protest.
Getzug said the action came two months prior to the expiry of an agreement on working conditions between the ports and workers and could be an attempt to leverage contract negotiations.
The West Coast ports are the nation's principal gateway for cargo container traffic from Asia. In a typical day shift, about 10,000 cargo containers are loaded and unloaded from ships coastwide, Getzug said.
Longshore workers handle everything from operating cranes at port marine terminals to clerical work like coordinating truck cargo deliveries.
A total of about 25,000 of them work at 29 ports in California, Oregon and Washington. About 6,000 were expected to be working the day shift yesterday, handling cargo from 30 ships coastwide, Getzug said.