Jailed activist Joshua Wong was taken to the Lai Chi Lok Reception Centre for questioning on Thursday after being arrested under the national security law, a post on his Facebook account said.
Wong, 24, is believed to be suspected of subversion, along with more than 50 other pro-democracy figures caught up in a wave of arrests on Wednesday.
The post said Wong, who's serving time for a protest outside police headquarters in 2019, was questioned without a lawyer present.
He gave a statement at the reception centre before he was taken back to Shek Pik Prison to continue serving the 13.5-month sentence he was handed last month for organising an unauthorised assembly and inciting others to take part in it.
Meanwhile, sources said Tam Tak-chi, more commonly known as "Fast Beat", was also informed of his arrest at the Lai Chi Kok Reception Centre where he's being held for sedition charges, pending a trial in May.
Earlier, some of those arrested on Wednesday were released on bail without being charged, including American lawyer John Clancey.
Separately, the deputy head of the Public Opinion Research Institute, Chung Kim-wah said he had visited police headquarters to assist with the force's investigation.
The institute had helped organise the pro-democracy camp's primary Legco polls last July, which are at the heart of the police's case. But Chung said he had not given any information to officers.
He added that even though authorities carried out a search of his home in Yuen Long, they did not take away any documents.