A woman from Hong Kong who was studying in Japan was arrested last month when she briefly returned home, on suspicion of violating the national security law for the Chinese territory.

Police in Hong Kong said on March 8 that they arrested the 23-year-old resident on suspicion of posting online messages inciting Hong Kong's independence and secession from China.

Sources close to the matter said she posted the messages on social media from 2019 through 2021, while she was studying in Japan.

She was later released on bail, but was ordered to submit her passport to the authorities and has since been unable to travel back to Japan.

The national security law that took effect in 2020 criminalizes four types of acts regarded as endangering national security. They are secession, subversion, organization and perpetration of terrorist activities, and collusion with a foreign country or with external elements.

The law also covers activities outside Hong Kong. It targets foreign nationals as well.

A professor at a graduate school of the University of Tokyo, Ako Tomoko, who is an expert on China, said the case is likely to mark the first time the law was applied to an act carried out in Japan. She noted that the law would undermine freedom of speech. Ako said people in Japan should take seriously the threat of the law being applied outside Hong Kong as well.