The anti-vaccine protest that temporarily cut off access to a mass vaccination site at Dodger Stadium was organized on Facebook through a page that promotes debunked claims about the coronavirus pandemic, masks and immunization.

The Facebook page, “Shop Mask Free Los Angeles,” issued a call last week to gather Saturday at the baseball park. Health authorities have been administering shots to as many as 8,000 people a day at the site, one of the largest vaccination centers in the country. Such venues form a critical component of the effort to corral the pandemic, which has lashed Los Angeles County so brutally in recent weeks that oxygen for patients has been in short supply.

The online activity illustrates the extent to which Facebook remains a critical organizing tool of the anti-vaccine movement, despite the company’s repeated vows to curb coronavirus misinformation and its assurance that it has removed more than 12 million pieces of such content. It also shows how social networking services could foster more confrontational tactics by those committed to false ideas about the dangers of immunization as the mass vaccination effort ramps up.

“I’m concerned this is the next phase of their anti-vaccine activism, going to places where the vaccine is being distributed and being disruptive there,” said Tara C. Smith, an epidemiologist at Kent State University and an authority on vaccine resistance.

Such activity, she said, could come to resemble protests at abortion clinics or demonstrations against stay-at-home orders at state capitols, a prospect that worries public health officials as they aim to speed vaccinations in a race against more-transmissible and possibly more-lethal variants now confirmed in this country.

Facebook spokeswoman Dani Lever said the company was reviewing the page and would “take action against any content that violates our policies.”

“Shop Mask Free Los Angeles” publicizes opportunities for “maskless shopping,” in violation of state and local rules requiring face coverings outside the home. It posts videos of confrontations inside businesses in Southern California, where anti-mask extremists film themselves going toe-to-toe with other customers and law enforcement. In the videos, the individuals behind the smartphones rail against what they see as “discrimination” and “medical tyranny.” A message sent to the page went unanswered.

The page itself has only about 3,000 followers, but the notice about what it termed a “PROTEST/MARCH” at the mass vaccination site was shared extensively in Facebook groups and on pages fixated on false ideas about masks, such as that they restrict breathing and that the Constitution forbids mandating their use. Names of the online forums include “Anti-Mask REVOLUTION!” and “Unmask California.”

The technology giant committed at the end of last year to enhancing its policies against coronavirus-related misinformation. That included a pledge to remove misinformation about the safety, efficacy, ingredients and side effects of coronavirus vaccines, as well as to prohibit ads that discourage vaccinations.

In a sign of gaps in the company’s enforcement, however, the “About” section of the anti-mask page promoting the Saturday protest included a link to a website devoted to the baseless “Plandemic” narrative accusing shadowy elites of enriching themselves by engineering the coronavirus and a vaccine for it.

The 26-minute documentary introducing the groundless theory went viral on Facebook in the spring before the company moved to erase it from its platform. When a follow-up came out in August, Facebook blocked users from posting it.

But the anti-mask page directs users to a “Planned-demic” website that features a wide range of videos devoted to the misinformation. Some of the links to the videos are broken. The site elevates claims by the video’s central figure, Judy Mikovits, a former scientist at the National Cancer Institute who has made a series of discredited claims.

Ellsworth S. Fortman, assistant Los Angeles fire chief, said that while scorching temperatures and high winds have interfered with testing and other aspects of the pandemic response, he never expected anti-vaccine activists would thwart other people’s access to inoculations.

In addition to the demonstration’s promotion on Facebook, Fortman said, in the days leading up to the protest he also received word about its advertisement on Instagram, which is owned by Facebook. One post transmitted to the department, which he shared with The Washington Post, came from an account called “socaltrumptrain.” It announced plans for an “anti-lockdown freedom rally and march” at Dodger Stadium.

The protest included only about 50 people, who waved signs and shouted anti-vaccine slogans. But their presence was sufficiently disruptive to force authorities to close the stadium’s gates, delaying thousands of motorists in line for a vaccine. Ultimately, it did not affect the number of people who were able to receive shots, officials said. No arrests were made.

Especially in Los Angeles, a hot spot for the coronavirus, any effort to throw up barriers to vaccination threatens to undermine the pandemic response, said Diana Shiba, president of the Los Angeles County Medical Association. The disruption, she said, could not come at a worse time, with medical workers racing to stay ahead of virus variants.

“It’s so unfair to those who want to receive a vaccine,” she said.

Meryl Kornfield contributed to this report.