According to police, posts the group shared online had called on people to join the classes to learn martial arts and the use of weapons to prepare themselves for a future revolution against the political powers that be.
Senior Superintendent Steve Li Kwai-wah, of the force’s National Security Department, said the aim of the training classes was “very subversive”.
He said participants were asked to “get themselves prepared and target the Chinese Communist Party” when the time came.
Li said there was a prima facie case to investigate whether those who joined the class had committed national security offences.
He added that the investigation against the group was ongoing, and officers were trying to locate the others who had taken part.
On Tuesday afternoon, the mixed martial arts coach and his assistant were formally detained on suspicion of committing acts with seditious intent.
Li said more evidence had been gathered since their arrests on Sunday, and police would discuss with the Department of Justice whether they should also face charges under the national security law.
During Sunday’s operation, police raided the Tsim Sha Tsui venue and the suspects’ flats in Sha Tin and Ma On Shan.
In the three premises, officers seized an airgun, eight crossbows, 30 arrows, three Japanese swords, six knives and two axes, along with HK$380,000 (US$48,560) in local and foreign currency.
Police said they were investigating whether the weapons had been used in illegal activities during the 2019 social unrest.
On Sunday, officers issued fixed-penalty tickets to the four other participants present at the venue – three men and one woman – for alleged breaches of pandemic-related social-distancing rules banning gatherings of more than two people.
Police said the group had also shared online posts urging people to set up a shadow government and a self-defence corps to overturn the Communist Party.
Senior Superintendent Li said the organisation had also called on people to flout the government’s anti-pandemic measures.
The national security law, which was imposed on Hong Kong by Beijing on June 30, 2020, outlaws secession, subversion, terrorism, and collusion with foreign or external forces.
The force’s National Security Department has arrested about 170 people for allegedly breaching the law since its imposition, and has charged more than 100 of them.