LOS ANGELES—West Coast cargo traffic came to a halt Thursday as port workers staged daylong anti-war protests to commemorate May Day, terminal operators said Thursday.

Thousands of dockworkers did not show up for the morning shift, leaving ships and truck drivers idle at ports from Long Beach to Seattle, Pacific Maritime Association spokesman Steve Getzug said.

Workers were expected to return for the start of the evening shift, he said.

"There's no work happening so that means there's no cargo being unloaded and certainly being loaded either," Getzug said.

The West Coast ports are the nation's principal gateway for cargo container traffic from the Far East.

J. Craig Shearman, a spokesman for the National Retail Federation, said shippers and exporters expected no significant, long-term disruptions from the walkout.

"This is something that happens every year," Shearman said. "Shippers and exporters know about it and plan around it, and we don't expect to see any significant disruptions from it."