National security police on Thursday said they had arrested four males, aged 15 to 47, on suspicion of conspiracy to commit subversion.

Police said they were believed to be members of a Taiwan-based organisation that promotes Hong Kong independence, provides military training and seeks foreign interference, with some of them described as "core members".

During an arrest operation on Wednesday, officers said they seized electronic devices detailing the activities of the organisation’s members, as well as a letter urging the US government to come up with a plan to rescue "political prisoners".

Police also said they confiscated flags, including one printed with the slogan "Free Hong Kong, Revolution Now", alongside others symbolising independence movements in Tibet, Xinjiang and Guangdong.

"According to the investigation, some of them [the suspects] are responsible for the design of the so-called flags for their organisation. Another one was to plan how to solicit assistance from foreign countries, and even one of them had their own plans to organise or to provide some military training for their fellow members," said Chief Superintendent Steve Li.

He also said the four suspects had communicated through social media and secure communication devices.

The chief superintendent warned that conspiracy to commit subversion carries a maximum penalty of life imprisonment.