(Reuters) - A U.S. law against Chinese-owned TikTok evokes the censorship regimes put in place by the United States' authoritarian enemies, free-speech advocates told the Supreme Court on Friday.

In an amicus filing, PEN America, Columbia University's Knight First Amendment Institute, and the Free Press urged the country's highest court to strike down the federal TikTok law to ban it or force its sale. They argue that not only did the law unlawfully threaten to restrict Americans from accessing foreign media in violation of the First Amendment of the U.S. Constitution, it "recalls practices that have long been associated with repressive governments."

The brief noted the Soviet Union and Chinese communists jammed Western radio broadcasts following World War Two, while modern-day Russia and China have a host of restrictions on websites and apps including Facebook, X, and YouTube.

TikTok and its owner ByteDance are fighting to keep the popular app online in the United States after Congress voted in April to ban it unless ByteDance sells it by Jan. 19.

The Justice Department has said that as a Chinese company, TikTok poses "a national-security threat of immense depth and scale" because of its access to vast amounts of data on American users. Judges have accepted the government's argument that it is acting solely to protect the U.S. from a "foreign adversary nation" and hamper Beijing's ability to gather data on Americans.

In their amicus brief, the free-speech advocates said any bid to protect Americans' data would best be achieved through privacy legislation, rather than banning a popular mode of expression.

The brief noted that, if successful, the U.S. would ironically join Beijing in banning TikTok. Chinese authorities only allow the app's domestic version, called Douyin, which is subject to heavy censorship.