LOS ANGELES — Thousands of dockworkers at 29 West Coast ports took the day off work yesterday, effectively shutting down operations at the busy complexes in what the union called a protest of the war in Iraq but employers worried might be a prelude to labor unrest.

The stand-down at ports including Los Angeles and Long Beach — which combined handle 40 percent of the imported goods arriving in the United States each year — idled ships and cranes, stranded thousands of big rigs and halted movement of about 10,000 containers during the eight-hour day shift.

The show of force by the International Longshore and Warehouse Union, which ended as workers reported for last night's shift at Southern California's twin ports, came two months before its contract expires with the Pacific Maritime Association, a group of cargo carriers, terminal operators and stevedore companies.

The action also, as one labor historian put it, added significant support for May Day, which has become the pre-eminent working-class and protest event of the year. The union might have taken a calculated risk that allowing its members to participate was worth potentially aggravating employers in the middle of contract negotiations.

"This union looks at itself as the vanguard of the working class on the West Coast, and I think there was a sense that they needed to participate in this event," said Nelson Lichtenstein, a University of California, Santa Barbara, history professor and director of the school's Center for the Study of Work, Labor and Democracy.

At the ports of Los Angeles and Long Beach yesterday morning, however, there were no anti-war activities — no protesters, no signs with anti-war sentiments and no indication of any large-scale opposition by dockworkers to U.S. policy in Iraq. The issue was discussed, union leaders said, during a private meeting of rank-and-file members at the ILWU Local 13 headquarters in Wilmington.

"We are supporting the troops and telling politicians in Washington that it's time to end the war in Iraq," President Bob McEllrath said in a news release.

The union's 25,000 members decided in early January to stand down on May 1. Their day off came despite an arbitrator's order Wednesday that they report to work. That order followed a Pacific Maritime Association complaint about the planned action, which it said violated contract obligations.

"Is this a voluntary war protest or a strike aimed at leveraging labor negotiation? We're not sure," said Steve Getzug, spokesman for the association. "We're concerned. We thought these kinds of old tricks were a thing of the past."

During negotiations in 2002, employers accused the union of a work slowdown and locked out the union from West Coast ports for 10 days, causing a retail business crisis that was interrupted when President Bush invoked the Taft-Hartley Act. At the time, economists estimated that the labor dispute cost the economy $1 billion to $2 billion a day.

"The arbitrator is the 'supreme court' of the waterfront and what he says has resonance," Getzug said. "And he said twice to the union that it had a duty to inform its membership to report to the docks today. The evidence is clear they defied that order."

The nation's largest retail group said it wasn't surprised by what happened yesterday.

"This is something that happens every year (and) shippers and retailers know about it," said Craig Sherman, the National Retail Federation's vice president of government relations. "It's going to have no impact at all in terms of merchandise on store shelves."

Nor was Sherman concerned about the implications for contract negotiations, which he said began earlier this year and are "going pretty smoothly."

Perhaps hardest hit by the job action were the local ports' 16,800 independent truck operators, many of whom were greeted at terminal gates by security guards with a blunt message: "We're closed. Turn around."

Among them was Guillermo Castillo, 35, of Calexico, who decided to wait it out near the TraPac Terminal in the Port of Los Angeles. Resting his head on a towel matted against his cab door, Castillo complained: "I heard nothing about this. I'm losing a whole day of work and about $580."