Hours before a new federal law banning TikTok from the United States was set to take effect, the social media app showed users a pop-up message that said it would be “temporarily unavailable” starting on Sunday.
“We regret that a U.S. law banning TikTok will take effect on January 19,” the message said. “We’re working to restore our service in the U.S. as soon as possible.”
TikTok, which is owned by the Chinese company ByteDance and has been under scrutiny for national security concerns, appeared to be readying its users for how it might go offline. At midnight, it was unclear if the app would still be available in U.S. app stores or how well it might run.
The law has a provision to penalize app store operators like Apple and Google, and internet hosting companies like Oracle for distributing or maintaining the TikTok app. Under the law, those companies face penalties as high as $5,000 per user who can access the app.
TikTok’s message to U.S. users followed the Supreme Court decision on Friday upholding the law, which calls for ByteDance to sell the app by Sunday or otherwise face a ban. The law had overwhelmingly passed Congress last year and been signed by President Biden. TikTok had expected to win its legal challenge to the law, but failed.
TikTok had made last-minute pleas to both the Biden administration and President-elect Donald J. Trump for a way out of the law. No one — including the U.S. government — was entirely sure what would happen to it when the law took effect. The United States has never blocked an app used by tens of millions of Americans essentially overnight.
For TikTok and ByteDance, the developments are a major blow. TikTok has roughly 170 million users in the United States, who are some of the app’s most lucrative customers. In legal filings, TikTok has said that even a temporary disappearance could kneecap it, with users and creators leaving for other platforms and never returning even if a ban was lifted.
The situation was further complicated by the law’s start date falling in the final days of Mr. Biden’s presidency. A White House spokeswoman suggested on Saturday that the Biden administration would not start fining companies on Sunday.
“We see no reason for TikTok or other companies to take actions in the next few days before the Trump administration takes office on Monday,” Karine Jean-Pierre, the White House press secretary, said in a statement. “We have laid out our position clearly and straightforwardly: actions to implement this law will fall to the next administration.”
Mr. Trump said on Saturday he would “most likely” find a way to give TikTok a 90-day extension once he takes office on Monday. The law gives the president the ability to extend the deadline for a sale only if there is “significant progress” toward a deal that would put TikTok in the hands of a non-Chinese owner. It was not clear how that extension might work if the ban had already taken effect.
On Saturday, the mood on TikTok was somber. Alix Earle, a content creator with 7.2 million followers who rose to fame on the app in 2022, posted tearful videos mourning the platform.
“I feel like I’m going Through heartbreak,” Ms. Earle wrote in one video. “This platform is more than an app or a job to me. I have so many Memories on here. I have posted every day for the past 6 years of my life. I’ve shared my friends, family, relationships, personal struggles, secrets.”
Ms. Earle added that she had been “in denial” about the ban.
