Truckers’ protests over a labor law shut Oakland’s port.

Most operations at the Port of Oakland, a major West Coast cargo hub, were shut down on Thursday as a convoy of truckers staged a fourth day of protests over a California labor law. The disruption threatened to further congest a global supply chain already hampered by delays.

The truckers, primarily independent owners and operators, are demonstrating in opposition to Assembly Bill 5, a law passed in 2019 that requires gig workers in several industries to be classified as employees with benefits, including minimum wage and overtime pay.

Since Monday, parked semitrucks have clogged thoroughfares leading to the Port of Oakland as truckers call on Gov. Gavin Newsom to address their concerns. Along with a coalition of trade groups, the truckers want Mr. Newsom to issue an executive order putting off the application of the 2019 law to their work and to bring labor and industry to the table to negotiate a path forward.

The governor’s office did not respond immediately to a request for comment for this article.

Smaller protests were organized last week at the twin ports of Los Angeles and Long Beach.

In a statement, Danny Wan, executive director of the Port of Oakland, said he understood the displays of frustration, but he warned against more delays surrounding the ports.

“Prolonged stoppage of port operations in California for any reason will damage all the businesses operating at the ports and cause California ports to further suffer market share losses to competing ports,” he said.

When Mr. Newsom signed the measure into law, it received immediate rebukes from companies like Uber and Lyft, whose leaders argued that the law would change their businesses so severely that it might well destroy them.

The state law codified a California Supreme Court ruling from 2018 that said, among other things, that people must be classified as employees if their work was a regular part of a company’s business.

Both Uber and Lyft, along with DoorDash, quickly lobbied for a ballot measure that would allow gig economy companies to continue treating their drivers as independent contractors.

California voters passed the measure, Proposition 22, in 2020, but last year a California Superior Court judge ruled that it was unconstitutional. Uber and Lyft quickly appealed and have been exempt from complying with Assembly Bill 5 while the court proceedings play out.

But that wasn’t the case for the truckers. In June, the U.S. Supreme Court declined to hear a challenge by California truckers, who under the new law are viewed as employees of the trucking companies they do business with.

Nearly 70,000 California truck drivers work as independent owners and operators, ferrying goods from ports to distribution warehouses. They argue that if Assembly Bill 5 is applied to them, they will have to find jobs with individual trucking companies or get licensing and insurance to operate as small businesses, increasing their out-of-pocket costs.

Matt Schrap, chief executive of the Harbor Trucking Association, a trade group for transportation companies serving West Coast ports, said the “frustration is that there is no pathway for folks to have independence.”

“That frustration is boiling over into action,” Mr. Schrap said.

Lorena Gonzalez Fletcher, a former state lawmaker who was an architect of the labor bill, said trucking companies are not being candid with drivers.

“These truck companies have a business model that is misclassifying workers,” said Ms. Gonzalez Fletcher, who is about to take over as head of the California Labor Federation. “How they have been operating has been illegal.”

The trucker protests come as the International Longshore and Warehouse Union is engaged in contract negotiations with the Pacific Maritime Association, representing the shipping terminals at 29 ports from San Diego to Seattle. Slowdowns among the unionized dockworkers could lead to global delays for a supply chain already hemorrhaging from Russia’s invasion of Ukraine and Covid-19 lockdowns in China.

On Thursday, German Ochoa, a trucker who lives in Oakland, arrived at the port, just as he had every day this week.

As horns from semitrucks blared in the background, Mr. Ochoa said by phone that he was standing shoulder to shoulder with other truckers. Some held poster boards that read, “Take down AB 5!!!” and “AB 5 Has Got to Go!,” he said.

“This is taking away my independence,” Mr. Ochoa said. “It’s my right to be an independent driver.”