An anti-hate speech campaign group has accused Elon Musk’s X Corp of intimidation after the owner of the rebranded social media site X, formerly known as Twitter, threatened legal action over the organisation’s research into hate speech on the platform.

The Center for Countering Digital Hate (CCDH) has regularly conducted research into the platform’s content since it was bought last year by Musk and has produced work claiming publication of hateful material on the site has risen since the $44bn (£34.2bn) deal was completed.

Musk’s legal representative has written to CCDH and its chief executive, Imran Ahmed, accusing the organisation of posting articles making “inflammatory, outrageous, and false or misleading assertions about Twitter”. This month Musk announced that Twitter would be rebranded as “X”, under plans to turn the platform into a Chinese-style super-app.

The letter was sent by Alex Spiro of US law firm Quinn Emanuel Urquhart and Sullivan, one of Musk’s main lawyers who also sent a legal warning to Meta this month over its launch of Threads – the “Twitter killer” app.

The letter to CCDH also threatens legal action, stating that X is considering whether what it describes as CCDH’s “false and misleading claims” are actionable under the Lanham Act, a piece of US legislation that covers trademark law. It adds: “Please be advised that Twitter will employ any and all legal tools at its disposal to prevent false or misleading claims from harming its users, platform, or business.”

CCDH’s legal representative said the “ridiculous” letter was an attempt to “intimidate those who have the courage to advocate against incitement, hate speech and harmful content online”.

In the letter, Spiro accused CCDH of making a “series of troubling and baseless claims that appear calculated to harm Twitter generally, and its digital advertising business specifically”. In its last published set of annual results, advertising accounted for 90% of Twitter’s $5.1bn in revenue but advertisers have held back spending since the takeover, some have argued this is because of concerns over Musk’s ownership and content moderation standards. Musk said recently that advertising revenue at Twitter had fallen by 50%.

Spiro singled out a CCDH article claiming that “Twitter fails to act on 99% of Twitter Blue accounts tweeting hate,” which it says was based on CCDH staff reporting 100 tweets and checking whether action had been taken against them four days later. Spiro said the article provided no methodology for its selection or testing of tweets and did not explain why the 100 tweets – out of nearly 500m sent a day from the platform – were representative of Twitter’s content moderation practices.

“The article is little more than a series of inflammatory, misleading, and unsupported claims based on a cursory review of random tweets,” Spiro wrote. Spiro also alleged that CCDH’s operations were “supported by funding from X Corp’s commercial competitors, as well as government entities and their affiliates”. CCDH said it did not receive funding from social media companies or government bodies.

Replying to Spiro’s letter, Roberta Kaplan of US law firm Kaplan Hecker and Finck said CCDH work did not represent the kind of advertisement or commercial speech that might trigger the Lanham Act, adding that the legal threat was an attempt to silence criticism and was at odds with Musk’s self-avowed commitment to free speech.

“Simply put, there is no bona fide legal grievance here. Your effort to wield that threat anyway, on a law firm’s letterhead, is a transparent attempt to silence honest criticism. Obviously, such conduct could hardly be more inconsistent with the commitment to free speech purportedly held by Twitter’s current leadership.”

Kaplan added that examples of the 100 Twitter Blue posts included: “Black culture has done more damage [than] the [Ku Klux] Klan ever did.”

Ahmed said: “Musk is targeting CCDH because we reveal the truth about the spread of hate and disinformation on Twitter under his ownership, and it’s impacting his bottom line.”

CCDH was offered support by the Molly Rose Foundation, set up by the family of Molly Russell, a British teenager who took her own life in 2017 after viewing harmful content online. The foundation’s chair of trustees, Ian Russell, Molly’s father and also a CCDH UK board member, said: “Elon Musk’s legal threats amount to an unprecedented attack on civil society and set a dangerous precedent for tech companies to seek to intimidate and silence independent tech accountability campaigners.”

Prof Brian Quinn from Boston College Law School said: “We still have a first amendment here in the US. It’s not against the law to say out loud that X is poorly managed by people who appear to have almost no self-control.”