Tony Chung, a former leader of a student group that called for Hong Kong independence, was arrested on Tuesday morning by members of the police's national security department, sources said.
Media reports said he was arrested in Central, near the US Consulate.
Chung's former group, Studentlocalism, later said on social media that two other ex-members, Yanni Ho and William Chan, had also been detained, while reporting to police stations.
The three had also been arrested under the national security law back in July, on suspicion of inciting secession.
Studentlocalism posted on its Facebook page on Tuesday that Chung had been missing since around 8am.
"He was not found at his home, with no trace as to where he is," the post said, adding that he was meant to report to a police station.
Subsequent posts said Ho had been arrested at Tin Sum Police Station and Chan at Tuen Mun Police Station, and that Chung was also being questioned by the police.
Studentlocalism announced in June that it was ceasing its activities in Hong Kong in light of the national security law, but members overseas would continue their work.
Last updated: 2020-10-27 HKT 14:44