Photo/Illutration People take to the streets to protest against the extradition bill, carrying a banner that reads, “There’s no riots, only tyranny” in Hong Kong on July 7, 2019. (Yuichiro Masumitsu)

TAIPEI--Hong Kong authorities arrested a student over political messages she posted on social media from Japan--the first known enforcement of its controversial National Security Law over activities that occurred outside its territory.

Hong Kong police told the student in her 20s that she was suspected of violating the law by taking part in separatist activities, according to sources in Taiwan and Japan who are close to the student.

The student was arrested in early March when she returned from Japan to update her ID.

She has been released but remains under investigation and her passport was confiscated. Hong Kong authorities are expected to give her further notice as early as May, according to the sources.

A former Hong Kong assembly member said the arrest is the first known instance of the National Security Law, introduced by China in 2020, being enforced over an activity done outside Hong Kong.

The former assembly member, now living in Taiwan to avoid being arrested under the law, said the same thing could happen to foreign citizens in the future.

“We need to be acutely aware of how the National Security Law could undermine freedom of speech in Japan,” said Tomoko Ako, a professor at the University of Tokyo. 

The student had stayed in Japan since 2019, when an extradition bill proposed by Beijing sparked widespread protest in Hong Kong. 

She posted online messages on the region’s independence following the protest.