Hong Kong democracy leader Joshua Wong is among three activists who have been arrested ahead of another weekend of planned protests.
Wong's political party Demosisto said he was pushed into a private car and escorted to police headquarters in Wan Chi on Friday.
The group said they were seeking further information on the arrest, which comes ahead of a major rally tomorrow.
Andy Chan, a founder of the pro-independence Hong Kong National Party that was banned last September, said on his Facebook page late on Thursday he had been detained at Hong Kong's international airport and that he was told he was about to be arrested.
And Agnes Chow, a member of Wong's politcal party Demosisto, was also arrested, the group said. It was not clear what charges she faced.
Hong Kong is grappling with its biggest political crisis since its handover to Beijing more than two decades ago.
Wong is the face of movement for full democracy that held protests in 2014, paralysing parts of the city for 79 days.
He was released from jail in June after serving a five-week term for contempt of court.
Demosisto, which advocates for greater democracy in Hong Kong, said: "He was suddenly pushed into a private car on the street."
In a statement on its official Twitter account it added: "He has now been escorted to the police headquarters in Wan Chai," it said, adding its lawyers were working on the case.
Police did not immediately respond to a request for comment.
Unrest in Hong Kong escalated in mid-June over a now-suspended extradition bill that would have allowed people to be sent to mainland China for trial in Communist Party-controlled courts.
It has since evolved into calls for greater democracy under the "one country, two systems" formula, which guarantees freedoms not enjoyed on the mainland, including an independent judiciary.
On Thursday, China brought fresh troops into Hong Kong in what it described as a routine rotation of the garrison.
Chinese state media stressed the troop movement was routine and Asian and Western diplomats watching the People's Liberation Army (PLA) forces had been expecting it.
Police have refused permission for a pro-democracy march on Saturday, but organisers have appealed against the decision.
The protest would mark five years since Beijing ruled out universal suffrage for Hong Kong and comes as Hong Kong faces its first recession in a decade, with all its pillars of growth under stress.
