The leader of the banned Hong Kong National Party, Chan Ho-tin, says he was detained at the airport on Thursday night while preparing to board a flight.
Writing on FaceBook, he said he only knew that the detention was requested by the local police, but the reason was unknown.
The police on Friday morning issued a statement confirming that a 29-year-old man surnamed Chan was arrested on Thursday night at the airport on suspicion of taking part in a riot and assaulting police. It said he was still being held for questioning.
Chan was among eight people arrested for possession of offensive weapons following a raid in Sha Tin earlier this month.
Officers swooped on the Haribest Industrial Building on Au Pui Wan Street, where they claimed they found materials for making petrol bombs and other weapons.