The company claimed that members of the Global Alliance for Responsible Media (GARM) coordinated to dissuade brands from advertising on X.

On Tuesday, X filed a lawsuit against GARM, a coalition of major advertisers, alleging that it violated antitrust laws by collaborating with brands to discourage them from spending money on the social media platform. The suit, filed in federal court in Texas, claims that GARM “conspired” with prominent brands, including CVS, Unilever, Mars, and the Danish energy company Ørsted, to “collectively withhold billions of dollars in advertising revenue” from X, formerly known as Twitter, following Elon Musk’s acquisition of the company in 2022.

“The illegal behavior of these organizations and their executives cost X billions of dollars,” wrote Linda Yaccarino, X’s chief executive, in an open letter to advertisers. “People are hurt when the marketplace of ideas is undermined, and some viewpoints are not funded over others as part of an illegal boycott.”

With the lawsuit, X effectively declared war on advertisers, which provide the bulk of the social media company’s revenue. Since Mr. Musk acquired the company and promised to usher in a new era of unfettered free speech, many advertisers have limited their spending on X, concerned by reports of rising hate speech and misinformation. By pursuing legal action against GARM, Mr. Musk continued to break with the leaders of other social media companies, who have forged close relationships with advertisers and been responsive to their concerns about offensive online content.