Police arrested nine people – five men and four women – and seized explosive substances, foiling an alleged plan to blow up multiple sites in Hong Kong.

The suspects, aged between 15 and 39, are from a local radical separatist group, said police. Six of them are secondary school students, while one is a faculty member of a secondary school and one a management-level staff at a local university.

The police also raided a hostel in Tsim Sha Tsui, one of the city's most popular shopping districts, where the suspects rented a room that was allegedly turned it into a bomb-making laboratory.

Speaking at a press briefing on Tuesday, Li Kwai-wah, senior superintendent of Police National Security Department, said the seizure of the documents and brochures suggesting a concrete and well-organized action plan of the radical group shocked the law enforcers.

According to police's preliminary investigation, the group planned to instigate secondary students in early July to carry out bomb attacks at public places, including the Cross-Harbour Tunnel, courtrooms and railway stations with self-made bombs, and then to send them away from the city.

The explosives seized are called triacetone triperoxide or TATP, which is often used in many terrorist attacks overseas, senior superintendent Li said, adding that they can have extensive impact. Police investigation will continue, he added.