Agnes Chow, 27 - a core member of a now-disbanded group of younger activists including Joshua Wong who helped drive Hong Kong's pro-democracy movement - said in posts on her Instagram account late on Sunday that she had left Hong Kong for studies in Canada.
Their group, Demosisto, dissolved hours after Beijing passed a sweeping national security law in 2020.
Beijing says the law has brought stability to Hong Kong after mass pro-democracy protests in 2019.
There was no immediate response from the Hong Kong police.
Chow was due to report to Hong Kong's security police later this month but decided not to return out of consideration for her personal safety and wellbeing.
Persons:
Agnes Chow, Tyrone Siu, Hong, Joshua Wong, Chow, Jimmy Lai, hadn't, Chow couldn't, David Holmes
Organizations:
REUTERS, Hong, Reuters, Hong Kong, Thomson
Locations:
Hong Kong, China, HONG KONG, Canada, Beijing, Toronto, Shenzhen, Hong