Thirteen US states and Washington DC are suing TikTok over claims it is harming children's mental health and not doing enough to protect them.
The lawsuits allege the video-sharing app is designed to be addictive and keep teenagers glued to the screen.
TikTok said the claims were "inaccurate and misleading" and pointed to features such as default screen time and privacy settings for under-16s.
The legal action is another blow for the app, owned by Chinese firm Bytedance, which already faces a potential US ban over fears it could give data to the Beijing government - something it insists will not happen.
"Young people are struggling with their mental health because of addictive social media platforms like TikTok," said New York attorney general Letitia James.
She also alleged young people had died and been injured copying stunts they had watched.
Her counterpart in Washington DC, Brian Schwalb, called it "an intentionally addictive product".
His lawsuit accuses TikTok of causing "profound psychological and physiological harms" - including depression, anxiety and body dysmorphia.
Other claims in the mass legal action include that a "virtual strip club with no age restrictions" is effectively able to operate via TikTok's live streaming and virtual currency functions.
TikTok spokesperson Alex Haurek said he was dismayed the states had not chosen to work with the service on their concerns.
"We strongly disagree with these claims, many of which we believe to be inaccurate and misleading," he said.
"We're proud of and remain deeply committed to the work we've done to protect teens and we will continue to update and improve our product.
"We've endeavoured to work with the attorneys general for over two years, and it is incredibly disappointing they have taken this step rather than work with us on constructive solutions to industry-wide challenges."
TikTok provides safety features including default screen time limits and privacy defaults for users under the age of 16, the company said.
TikTok also doesn't allow under-13s to use its main service and restricts some content for under-18s.
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The cases filed on Tuesday stem from an investigation launched by a bipartisan coalition of prosecutors in March 2022.
Those suing under the new action are: California, Illinois, Kentucky, Louisiana, Massachusetts, Mississippi, New Jersey, New York, North Carolina, Oregon, South Carolina, Vermont, Washington state, as well as Washington DC.
Other US states have previously launched similar child protection cases against TikTok.
In August, the US Justice Department also sued the app at a federal level for allegedly failing to protect children's privacy.
However, the main threat remains a new US law which threatens to ban TikTok in the new year unless Bytedance sells it.
The company has appealed against the ruling and judges are expected to issue a decision - which could ultimately end up in the Supreme Court.
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