Demosisto members Joshua Wong and Agnes Chow were arrested on Friday morning.
Police said Wong was arrested on suspicion of organising, taking part and inciting others to join an unauthorised assembly on June 21, when protesters surrounded police headquarters in Wan Chai.
Chow is suspected of taking part and inciting others to join an unauthorised assembly.
The former leader of the now-banned Hong Kong National Party, Chan Ho-tin, meanwhile, said he was detained on Thursday evening at the airport as he was preparing to board a flight.
“The ludicrous dawn swoops by police to arrest Agnes Chow and Joshua Wong are an outrageous assault on the rights to freedom of expression and peaceful assembly,” said Amnesty International Hong Kong’s director Tam Man-kei, adding that they must be released on bail as soon as possible.
Tam said that the past week has seen “scare tactics straight out of Beijing’s playbook”, and referred to attacks on pro-democracy protest organisers, the banning of a major rally planned for Saturday as well as the arrests.
“The authorities must end this concerted attack on the freedoms of expression and peaceful assembly,” Tam said. “It is vital that the authorities send a clear message that people in Hong Kong can still enjoy these rights, irrespective of their political beliefs.”