WASHINGTON, Sept 16 (Reuters) - A lawyer for TikTok and

Chinese parent company ByteDance faced tough questions on Monday

as a U.S. appeals court heard arguments in their lawsuit seeking

to block a law that could ban the short video app used by 170

million Americans as soon as Jan. 19.

A three-judge panel of the U.S. Court of Appeals for the

District of Columbia began hearing arguments in the suit filed

by TikTok and ByteDance in May seeking an injunction barring the

law from taking effect.

The judges questioned TikTok's outside lawyer Andrew Pincus,

who argued that the U.S. government had not demonstrated that

TikTok poses national security risks and that the law violates

the U.S. Constitution on a number of grounds including running

afoul of First Amendment free speech protections.

"The law before this court is unprecedented, and its effect

would be staggering," Pincus told the judges, saying "for the

first time in history, Congress has expressly targeted a

specific U.S. speaker, banning its speech and the speech of 170

million Americans."

The law gives ByteDance until Jan. 19 to sell or divest

TikTok's U.S. assets or face a ban in the United States. Driven

by worries among American lawmakers that China could access data

on Americans or spy on them with the app, the U.S. Congress

passed the measure with overwhelming support and President Joe

Biden signed it into law in April.

The law prohibits app stores like Apple ( AAPL ) and

Alphabet's Google from offering TikTok and bars

internet hosting services from supporting TikTok unless

ByteDance divests TikTok by the deadline. Under the law, Biden

could extend the deadline by three months if he certifies

ByteDance is making significant progress toward a sale.

Judges Sri Srinivasan, Neomi Rao and Douglas Ginsburg were

hearing the arguments.

The case is playing out during the final weeks of the U.S.

presidential campaign. Republican presidential candidate Donald

Trump and Vice President Kamala Harris, his Democratic rival,

are active on TikTok, seeking to court younger voters.

The Justice Department has said TikTok, under Chinese

ownership, poses a serious national security threat because of

its access to vast amounts of personal data on Americans,

asserting China can covertly manipulate information that

Americans consume via TikTok.

ByteDance has said divestiture is "not possible

technologically, commercially or legally."

TikTok and the Justice Department have asked for a ruling by

Dec. 6, which could allow the U.S. Supreme Court to consider any

appeal before a ban takes effect.

The White House has said it wants to see Chinese-based

ownership ended on national security grounds, but not a ban on

TikTok. Trump, who unsuccessfully tried to ban TikTok in 2020,

has said if elected in November he would not allow TikTok to be

banned.