TikTok restored service in the United States Sunday after briefly going dark, as a law banning the wildly popular app on national security grounds came into effect. 

TikTok credited President-elect Donald Trump, who retakes power on Monday, for making the reversal possible — though the outgoing administration of President Joe Biden had earlier said that it would not enforce any ban. 

The video-sharing app had shut down in the United States late Saturday as a deadline for its Chinese owners ByteDance to sell its US subsidiary to non-Chinese buyers loomed. 

Earlier Sunday, as millions of dismayed users found themselves barred from the app, Trump promised to issue an executive order delaying the ban to allow time to "make a deal."

He also called in a post on his Truth Social platform for the United States to take part-ownership in TikTok.

The president-elect said he "would like the United States to have a 50% ownership position in a joint venture," arguing that the app's value could surge to "hundreds of billions of dollars — maybe trillions."

"By doing this, we save TikTok, keep it in good hands," wrote Trump, who had previously backed a TikTok ban and during his first term in office made moves towards one.

In a statement posted on X following Trump's comments, TikTok said it "is in the process of restoring service."

"We thank President Trump for providing the necessary clarity and assurance to our service providers that they will face no penalties providing TikTok to over 170 million Americans."

TikTok, which was back online in the United States by Sunday afternoon, did not address Trump's call for part American ownership of the app. 

The law allows for a 90-day delay of the ban if the White House can show progress toward a viable deal, but so far ByteDance has flatly refused any sale.

The Biden administration said it would leave enforcement of the law to Trump. 

From teenage dancers to grandmothers sharing cooking tips, TikTok has been embraced for its ability to transform ordinary users into global celebrities when a video goes viral.

But it is also rife with disinformation, and its Chinese ownership has long spurred national security fears, internationally as well as in the United States.

Sunday's blackout came after the US Supreme Court on Friday upheld the legislation banning it pending any sale.

Trump, who signed an executive order stepping up pressure on ByteDance to sell in 2020, has since credited the app with connecting him to younger voters.

It is unclear what the incoming president can do to lift the ban unless ByteDance ultimately sells, however.

"Congress wrote this law to be virtually president-proof," warned Adam Kovacevich, chief executive of industry trade group Chamber of Progress.

'I love TikTok'

Besides removing TikTok from app stores, the law requires Apple and Google to block new downloads, with the companies liable for penalties of up to $5,000 per user if the app is accessed.

Oracle, which hosts TikTok's servers, would also be legally obligated to enforce the ban.

Other social media platforms such as X were flooded with memes and comments mourning Sunday's temporary shutdown — though some posts were more cynical, with many pointing to Trump's own previous efforts to ban TikTok.

In Europe, TikTok's suspension drew praise from the foreign minister of Estonia — among the countries most resilient to disinformation, according to the European Media Literacy Index. 

"Banning TikTok must be considered in Europe as well," Margus Tsahkna said on X.

The ban even became a hot topic at the Australian Open in Melbourne, where American tennis player Coco Gauff scrawled "RIP TikTok USA" on a courtside camera.

"I love TikTok, it's like an escape... hopefully it comes back," the world number three told reporters.

Offers for TikTok

A last-minute proposal made Saturday by the highly valued start-up Perplexity AI offered a merger with the US subsidiary of TikTok, a source with knowledge of the deal told AFP.

The proposal did not include a price but the source estimated it would cost at least $50 billion.

Frank McCourt, the former Los Angeles Dodgers owner, has also made an offer to purchase TikTok's US activities.