Tony Chung, the former leader of a now-disbanded Hong Kong student activist group, was detained by national security officers and arrested along with three others for allegedly inciting secession online on Wednesday, according to local media reports.
Chung, 19, and three other students aged between 16 and 21 were detained and arrested after a crackdown by national security officers, the first of its kind since China's new national security law was implemented at the end of June.
Senior Superintendent Li Kwai-wah of the newly-formed National Security Department told reporters the suspects were involved in an online platform calling for a "Hong Kong nation," according to local broadcaster RTHK.
Police refused four represented media groups to cover the police briefing saying that they were not registered with the government's information services department, government-backed RTHK reported.
The new national security law was gazetted into an annex of the city's own Basic Law on the eve of Hong Kong's 23rd handover anniversary of the ex-British colony back to China. The law targets secession, acts of terrorism, subversion and collusion with foreign forces.
A flurry of online activity followed Chung's arrest, with shocked district councillors and key activists such as Nathan Law, a former member of the political group Demosisto who is currently in self-imposed exile in Britain, posting the news on social media.
Beijing revealed its new national security headquarters at the Metropark Hotel in Causeway Bay, just one week after pushing the law through.
The movement began in response to a now withdrawn extradition law, which would have seen dissidents facing trial in mainland China.
Britain, the United States, Canada and New Zealand have since withdrawn their own extradition laws with Hong Kong over Beijing's new security law.