Police said Wong and Agnes Chow are being investigated for their role in a June 21 unauthorised protest outside a police station. Both are charged with participating in the demonstration and inciting others to join it. Wong is also charged with organising it.
Wong is secretary-general of Demosisto and Chow is a prominent member. He was one of the student leaders of the Umbrella Movement, the major pro-democracy demonstrations in 2014.
Both were granted bail hours after their arrests.
Isaac Cheng, the vice chair of the group, said the arrests are an attempt to spread fear and "white terror" among Hong Kong residents.
He accused authorities of trying to identify leaders in a "leaderless" movement that has rocked Hong Kong for nearly three months. The Communist Party-ruled government in Beijing is pulling the strings and has misjudged the situation, he said, urging residents to continue protesting despite the risk of arrest.
Demosisto first reported the arrests on its social media accounts, saying Wong was pushed into a private car as he was heading to a subway station around 7:30 a.m. and was taken to police headquarters. It later said Chow had also been arrested, at her home.
Wong was released from prison in June after serving a two-month sentence related to that protest. He has been speaking out regularly in support of the pro-democracy protests that have racked Hong Kong this summer.
The protests were set off by extradition legislation that would have allowed suspects to be sent to mainland China to face trial and expanded to the general concern that China is chipping away at the rights of Hong Kong residents.
The extradition bill was suspended but the protesters want it withdrawn and are also demanding democracy and an independent inquiry into police actions against protesters.
Police said Chan was arrested under suspicion of rioting and attacking police.
- AP