LOS ANGELES, May 1 (Reuters) -- Ports along the U.S. West Coast, including the country's busiest port complex in Los Angeles, were shut most of Thursday as some 10,000 dock workers went on a one-day strike to protest the war in Iraq, port and union officials said.

Twenty-nine ports from San Diego to Washington state that handle more than half of U.S waterborne trade ground to a halt, but shipping experts said the economic costs of the walk-out would be limited.

As a new shift started late in the day, workers were showing up at ports, effectively ending the conflict.

"At ports in Los Angeles and Long Beach, longshoremen are reporting in," Steve Getzug, spokesman for the Pacific Maritime Association, which represents all 29 ports. "We've been told by the union local leadership that things will be returning to normal this evening."

The International Longshore and Warehouse Union said all 29 ports are back at work and spokeswoman Jennifer Sargent called the stoppage "just an eight-hour break from work."

By not showing up for the day shift, more than 1,000 longshoremen brought the normally bustling port of Long Beach, a hub for trade with Asia, to a complete standstill.

Paul Bingham, an economist with Global Insight, which tracks container volume and congestion at U.S. ports, said labor officials had alerted shippers and carriers.

"If this had come as a surprise it would have been a lot more serious in its impact," said Bingham, also noting that it was not peak season for shipping.

"This isn't like the West Coast port lockout in 2002 when we shut down the ports for 10 days," he added.

The ILWU said some 10,000 workers joined the anti-war protest, spurred in part by its belief that big shipping companies are profiting from the war.

"Longshore workers are standing down on the job and standing up for America," said ILWU International President Bob McEllrath. "We're supporting the troops and telling politicians in Washington that it's time to end the war in Iraq."

'LEVERAGE CONTRACT NEGOTIATIONS'

But port officials cast doubts over the war protest motive.

PMA's Getzug said the action came two months prior to the expiration of the current labor agreement.

"Today's actions raised the question of whether this was an attempt to leverage contract negotiations," he said in a statement.

He added that the work stoppage was illegal under the PMA's contract with the ILWU.

It was not clear how many ships or containers were affected by the longshore workers action. But the PMA said that on a typical weekday shift between 10 a.m. to 5 p.m. about 10,000 containers are moved on the West Coast.

Arly Baker, spokesman for the Port of Los Angeles, said 15 ships were to arrive at the port on Thursday and about half of them had arrived and berthed before the work stoppage began for the day.

"What this amounts to is probably the same effect of an official holiday where the terminals shut down," Baker said. "There won't be a backup in cargo or some kind of bottleneck resulting from it."

Together, the neighboring ports of Los Angeles and Long Beach handle 43 percent of the container cargo imports, including most of the household goods shipped from China.

The two ports bring in about $1 billion of cargo daily, Baker said.

By: Jill Serjeant and Bernard Woodall

(Additional reporting by Alexandria Sage in Los Angeles, Adam Tanner in San Francisco and Dan Whitcomb in Long Beach; Writing by Mary Milliken)