New York. Millions of TikTok users in the United States were no longer able to watch or post videos on the social media platform as a federal ban on the immensely popular video-sharing app took effect Sunday amid questions over how long the blackout might last.

The company's app was removed Saturday night from prominent app stores, including the ones operated by Apple and Google. Apple told customers with its devices that it also took down other apps developed by TikTok's China-based parent company, including one that some social media influencers had promoted as an alternative.

“A law banning TikTok has been enacted in the US,” a pop-up message informed users who opened the TikTok app and tried to scroll through videos. “Unfortunately that means you can’t use TikTok for now.”

The federal law required TikTok's parent company ByteDance to cut ties with the platform's US operations by Sunday due to national security concerns posed by the app's Chinese roots. It passed with wide bipartisan support in April, and US President Joe Biden quickly signed it. TikTok and ByteDance sued on First Amendment grounds, and the US Supreme Court upheld the statute on Friday.

However, the service interruption TikTok instituted hours early caught most users by surprise. Experts had said the law as written did not require TikTok to take down its platform, only for app stores to remove it. Current users were expected to continue to have access to videos until the app stopped working due to a lack of updates.

The Biden administration stressed in recent days that it did not intend to implement or enforce the nationwide ban before President-elect Donald Trump takes office on Monday. Trump, who once favored a TikTok ban, said in an NBC News interview on Saturday that he was thinking about granting ByteDance a 90-day extension to find an approved buyer for the app's US operations.

“We are fortunate that President Trump has indicated that he will work with us on a solution to reinstate TikTok once he takes office. Please stay tuned,” read the pop-up message the app's users now see under the headline, “Sorry, TikTok isn’t available right now.”

The only option the message gives to US users is to close the app or click another option leading them to the platform's website. There, users are shown the same message and given the option to download their data, an action that TikTok previously said may take days to process.

Apple said in a statement on its website that three TikTok apps and eight other ByteDance-created apps were no longer available in the US, while visitors to the country might have limited access. The removed apps included video-editing program CapCut, art editing program Hypic and Lemon8, a video-sharing app that includes some of the same features as TikTok.

“Apple is obligated to follow the laws in the jurisdictions where it operates,” the company said.

Apple said the apps would remain on the devices of people who already had them installed, but in-app purchases and new subscriptions no longer were possible and that operating updates to iPhones and iPads might affect the apps' performance.

The federal law banning TikTok allows the sitting president to extend Sunday's deadline by 90 days if a sale is in progress. But no clear buyers have emerged, and ByteDance previously said it would not sell TikTok.

Trump told NBC News on Saturday that if decides to grand such an extension, it would “probably” be announced Monday after he is sworn in as president. TikTok CEO Shou Chew is expected to attend Trump’s inauguration with a prime seating location.

Chew posted a video late Saturday thanking Trump for his commitment to work with the company to keep the app available in the US and a “strong stand for the First Amendment and against arbitrary censorship.”

“We are grateful and pleased to have the support of a president who truly understands our platform. One who has used talk to express his own thoughts and perspectives, connecting with the world and generating more than 60 billion views of his content in the process,” Chew said.

On Saturday, artificial intelligence startup Perplexity AI submitted a proposal to ByteDance to create a new entity that merges Perplexity with TikTok's US business, according to a person familiar with the matter.

Perplexity is not asking to purchase the ByteDance algorithm that feeds TikTok user’s videos based on their interests and has made the platform such a phenomenon.

Other investors have also been eyeing TikTok. “Shark Tank” star Kevin O’Leary recently said a consortium of investors that he and billionaire Frank McCourt put together offered ByteDance $20 billion in cash. Trump’s Treasury Secretary Steven Mnuchin also said last year that he was putting together an investor group to buy TikTok.