Police said the suspects planned to use explosives to bomb courts, cross-harbor tunnels, railways and even planned to put some of these explosives in trash bins on the street.
Three males and one female, aged 16 to 21, were detained, a police official said at an 11 p.m. news conference. All are believed to be students.
On July 2, police arrested a 24-year-old man at the airport on suspicion of stabbing an officer during the protest just hours after China's new security law was imposed.
The sweeping new Chinese security law that critics warn will dismantle the last vestiges of Hong Kong's autonomy came into effect Wednesday