Police would not name the group or specify motives but said that maps of central districts of Hong Kong had been found and warned anyone taking part in public gatherings to stay away from "violent protesters".
It comes as a series of rallies are planned ahead of a vote this week on a controversial political reform package which has divided the city and led to mass protests at the end of last year.
"During police inquiries someone claimed to be a member of a local radical organisation," said Au.
Au would not clarify how many of the group belonged to the organisation, but said "not all".
He described the group as a "local syndicate involved in manufacturing explosives".
The South China Morning Post and the Oriental Daily had earlier said those arrested were activists from pro-democracy "localist" groups, which have emerged in the wake of a battle over the government's electoral roadmap.
Chemicals were seized at an abandoned television studio in the eastern district of Sai Kung, with some detonated at the scene.
A house search later led to the seizure of ingredients which could be used to make TATP, "a form of high explosive", said Au.
Maps of the central neighbourhoods of Wan Chai and Admiralty were also found.
Both locations were at the heart of last year's mass pro-democracy rallies and the government complex is in Admiralty.
Daily rallies are planned outside the legislature this week ahead of a vote on the government's electoral roadmap, expected to take place by Friday.
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