Police officers at a news conference, held on June 15, arrange air rifles seized with explosives in Hong Kong. Police in Hong Kong arrested nine people in raids and seized suspected contraband, authorities said. [Roy Liu/China Daily]

Hong Kong authorities haven't ruled out further arrests after police said on Monday that they busted nine radical activists who were allegedly preparing to bomb sites around the city's center of government.

Samples of the explosive triacetone triperoxide and at least 11 kg of related substances were seized by officers of the Organized Crime and Triad Bureau in a series of Monday-morning raids, beginning at a derelict television studio that police say was used as a location for making bombs.

Nine Hong Kong residents - four women and five men aged 21 to 34 - have been arrested on charges of conspiracy to manufacture explosives. The police also seized formulas for making explosives and smoke grenades.

The police declined to say which radical group the nine are affiliated with.

The suspects include a graduate student, a teaching assistant, a construction worker, a technician and three unemployed people, police sources said.

Sources said the group planned a series of bombings in the city ahead of a historic vote by lawmakers on granting greater political rights for the special administrative region.

The plan has run into stiff opposition by opposition lawmakers and local groups of all stripes, including elements advocating independence.

The source said the nine were core members of an anti-China nativist group who coordinated online.

A 3-D printer, various bomb-making implements, Guy Fawkes masks, and stickers and paraphernalia with links to opposition lawmakers were seized as evidence along with more than a dozen air rifles. Police are checking to see if sidearms popular with war-gamers have been modified to make them lethal.

Au Chin-chau, chief superintendent of the organized crime bureau, said that police were monitoring the group in an ongoing investigation before going public on Monday morning, with two men arrested at the studio before a string of raids across the city hours later.

Au said maps seized from the alleged perpetrators included sections of the Wan Chai and Admiralty districts, but he declined to clarify whether the suspected targets were government installations, shopping malls, schools or parks, citing operational security.

The Admiralty district is host to the city's seat of local government and was the scene of monthslong opposition occupations last year. It also is the site of the main People's Liberation Army barracks in the city.

Radicals, including those affiliated with opposition political parties, have issued online calls for bombing campaigns against public institutions, including the Central Liaison Office and the funeral procession of a government ally.

tim@chinadailyhk.com