Officers said they had found explosives, chemicals that could be turned into TATP, and bomb-making equipment in a hostel room in Tsim Sha Tsui.
Helmets, shields, air guns and walkie-talkies were also found, police said, adding that they had frozen bank funds of around HK$600,000.
Senior superintendent Steve Li – from the force’s national security department – alleged that the suspects had intended to make bombs in the hostel room, but said the police action had thwarted their attempts.
Li said the group planned to launch car bomb attacks and place bombs in rubbish bins, alleging that they hoped to maximise the damage caused to society.
He claimed that the five male and four female suspects, aged between 15 and 39, belong to a pro-independence group called “Returning Valiant”.
“Six of them are secondary school students. They all have some planning at some stage to leave Hong Kong for good. So the group is particularly interested in this group and recruited them to join the plot to do the sabotage before their departure,” Li said.
Among those arrested was a member of staff from the management of a local university and a secondary school employee.
Li also gave an update on the national security investigation into a knife attack on a policeman in Causeway Bay on July 1.
He said an autopsy revealed that the alleged perpetrator had a three-centimetre wound to his heart after stabbing himself.
Li said officers found many newspapers in the man’s home that contained “fake news” inciting hatred.