X is suing a nonprofit group in US federal court over reports that hate speech has flourished at the platform formerly known as Twitter.

Elon Musk-owned X accused the Center for Countering Digital Hate (CCDH) of generating "flawed" studies that have cost the tycoon's tech firm tens of millions of dollars, a copy of the lawsuit showed.

However, the legal basis outlined in the suit filed in San Francisco late Monday was an accusation that the nonprofit violated X's terms of service in the way it accessed data for its reports.

CCDH chief executive Imran Ahmed on Tuesday contended that Musk's legal move is an effort to silence critics that comes "straight out of the authoritarian playbook."

Ahmed stood by the group's research, saying it shows that hate and disinformation are "spreading like wildfire" at Musk-run X.

The eccentric billionaire's lawsuit asks a court to grant X unspecified cash damages and to order CCDH to stop the way it has been getting data for its reports.

The suit accuses CCDH and its parent organization in Britain of being "activist organizations masquerading as research agencies."

X goes on to argue in the suit that the nonprofit improperly gained access to data so it could "cherry-pick" information to back reports showing the rebranded Twitter is rife with harmful content.

"Musk is trying to shoot the messenger who highlights the toxic content on his platform rather than deal with the toxic environment he's created," Ahmed said in a statement.

"Musk will not bully us into silence."

Since Musk bought Twitter for $44 billion last October, the platform's advertising business has collapsed as marketers soured on his management style and mass firings at the company that gutted content moderation.

In response, the entrepreneur has moved toward building a subscriber base and pay model in a search for new revenue.

Many users and advertisers alike have responded adversely to the social media site's new charges for previously free services, its changes to content moderation, and the return of previously banned right-wing accounts.

In December, Musk reinstated former president Donald Trump's Twitter account although Trump has yet to return to the platform.

X recently reinstated rapper and designer Kanye West around eight months after his account was suspended, according to media reports.

Last fall, West, who now goes professionally by Ye, posted an image that appeared to show a swastika interlaced with a Star of David, and Musk suspended the artist from the platform, which he had bought weeks earlier.