While labour contract negotiators took the day off Thursday, port police estimated that 6,000 people took part in the rally in Los Angeles and Long Beach, port spokeswoman Rachel Campbell said. Hundreds more rallied in Tacoma, Washington.
Contract talks were expected to resume Friday.
Earlier this month a federal mediator intervened in contract bargaining that began eight months ago but by fall had deteriorated into a blame game as goods languished on docks.
Longshoremen have been working without a contract since the previous one expired in July.
The companies operating port terminals assert that workers have slowed the rate at which they process containers brought from Asia aboard massive ships, adding to gridlock as mountains of imports dwell on dockside yards.
Employers started scaling back work crews this month, saying they won't unload ships at night and instead are focused on clearing the backlog from dockside yards. They say the strategy is working.
"I'm told that there's been some improvement in the last week," said Steve Getzug, a spokesman for the Pacific Maritime Association, which represents terminal operators and shipping lines.
Longshoremen dispute employer statistics that indicate their productivity has dropped and say employers are cutting back jobs to hurt workers and pressure negotiators for their union into a bad deal.
As for congestion, dockworkers say they're not culpable and cite broader problems with the supply chain, including a shortage of truck beds to haul containers from yards to distribution warehouses.
What's more, congestion at Los Angeles and Long Beach — by far the nation's largest ports — is not improving with the cutting back of crews, according to local officials with the International Longshore and Warehouse Union.
"The employers' action is punitive. It's a measure to hit the pocketbooks of the workforce. It has zero to do with relieving congestion," said Bobby Olvera Jr., president of the union's Local 13 branch.
Olvera and other union leaders participated in marches Thursday evening in Los Angeles and Tacoma, Washington.
A couple hundred dock workers and supporters attended the rally and march in Tacoma.
The demonstration was designed to tell the community what's happening with the slowdown and to allow longshore workers to support each other, Dean McGrath, president of the union's Local 23 in Tacoma, told The News Tribune.
"This is a scary fight for everyone," he said.
"We're extremely concerned about it," said one of the younger Tacoma dock workers, 31-year-old Kyle Guinn. "This is our future."
West Coast seaports handle about one-quarter of the nation's trade, as measured by dollar value, according to international trade expert Jock O'Connell of Beacon Economics. That means about $1 trillion worth of imports or exports cross West Coast docks annually.
Contact Justin Pritchard at http://twitter.com/lalanewsman.