In a Facebook post, Simon Cheng says he was pressed for information about the Hong Kong protests.
Measures are also being made to clear roadblocks outside the Cross-Harbour Tunnel, which has been shut for a week.
Lee Chi-cheung remains in a coma after confronting a masked group who had vandalised an MTR station in Ma On Shan.
Local media report that police arrested, stopped or searched students dressed in school uniform in Kowloon and the New Territories.
Lui Cheung Kwong Lutheran College cancelled classes after demonstrators shut down the school.
District council members negotiated with the police to leave the area.
Mask-wearing secondary students in Kwun Tong marched and blocked roads, while Tin Shui Wai MTR closed due to a fight.
The legislation calls for an annual report that the city has retained enough autonomy from China to keep its special trading rules.
Primary and secondary schools will reopen, but kindergartens and schools for children with disabilities will remain closed until Sunday.
Chief Executive Carrie Lam says police were told to let minors go home after having personal details recorded.