Wong, 22, convenor of Hong Kong Indigenous, was reported missing after publishing his high-profile “final message to Hongkongers” on February 11.
Police arrested him on Sunday after breaking into a flat in Tin Shui Wai where officers found amonium nitrate, PEG (polyethylene glycol), and silicon dioxide (silica gel) – chemicals that could be used to make explosives.
They also seized an extendable baton, HK$530,000 in cash and about 100 pills of suspected part one poison, and marijuana. A Guy Fawkes mask, protective gear used in war games and a computer and circuit boards were also found.
Wong was held on suspicion of being involved in the February 8 Mong Kok riot.
Hong Kong Indigenous, which Beijing has referred to as a “radical separatist organisation”, posted Wong’s arrest on Facebook. The group believes in “separation between Hong Kong and China” as well as “using force in resistance”.
Activist Ray Wong Toi-yeung, a key figure in the Mong Kok riot, was arrested on Sunday for inciting others to participate in the violence.
Another of the group’s convenors, Edward Leung Tin-kei will run for the Legislative Council by-election this month.
The group staged an outdoor rally on Saturday night in Sha Tin to pledge support for Leung, who has been charged with rioting over the Mong Kok violence.
Leung is contesting the upcoming race with Civic Party’s Alvin Yeung Ngok-kiu, social worker Nelson Wong Shing-chi, independent Lau Chi-sing, vice-chairman of the Democratic Alliance for the Betterment and Progress of Hong Kong Holden Chow Ho-ding, independent candidate Albert Leung Sze-ho and Sai Kung district councillor Christine Fong Kwok-shan.
The by-election was triggered by the resignation of Civic Party legislator Ronny Tong Ka-wah last year.