Police said on Tuesday that three men and three women had been arrested by national security officers for selling publications related to the 2019 social unrest in a Mong Kok market.
The six suspects, aged between 18 and 62, were detained on suspicion of doing an act or acts with seditious intention.
They were held in custody overnight.
The suspects are alleged to have produced and published a book about the anti-government protests in Hong Kong from June 2019 to February 2020, and promoted and sold it in the market from December last year to January.
Officers said they seized 43 books and other items after raiding the suspects' homes and a printing shop.