- Hong Kong anti-government protests
image copyrightAFPimage captionPolice presented items they'd seized during the arrest
Hong Kong police have arrested nine people including six high school children over an alleged terrorist plot.
Members of the group are aged between 15 and 39 years old.
Police said they had rented a room in a hotel to build bombs and planned to attack the city's courts, transport networks and streets.
The arrests come as Hong Kong's leader Carrie Lam warned against "illegal ideas" spreading through the city.
The group is being held under the national security law, which has been used to crush dissent after years of pro-democracy protests lead by mostly young activists.
Police said they seized the highly explosive chemical triacetone triperoxide (TATP) in a hotel room that was used as a laboratory for bomb-making equipment.
Some HK$600,000 ($77,000, £56,000) linked to the group was also traced and frozen.
Senior Superintendent Li Kwai-wah told reporters the group had planned "to attack some of the public facilities in Hong Kong, including the Cross-Harbor Tunnel, railways, court rooms and they even wanted to lay bombs in the rubbish bin on the street, with a view to maximize the damage caused to the society".
- Hong Kong's year under China's controversial law
- Hong Kong free media buckles under China pressure
Shortly before the arrests were announced, Hong Kong's Chief Executive Carrie Lam urged parents, teachers and religious leaders to monitor teenagers and report any suspected crimes.
At her weekly press conference, she said government departments shouldn't allow what she called "illegal ideas" to spread through the education system.
She also praised the national security law imposed on the territory by Beijing last year.
image copyrightGetty Imagesimage captionHong Kong's Chief Executive Carrie Lam insists the media can continue to operate freely
"For a long time, citizens have been exposed to wrong ideas, such as achieving justice through illegal means," Ms Lam told reporters.
She also criticised some people for mourning the death of a man who stabbed and wounded a policeman before taking his own life on 1 July.
The incident took place on the anniversary of the former colony's handover from British to Chinese rule and the 100 year anniversary of the founding of China's Communist Party.
In the past, large protests against Beijing's growing influence would be organised on the day of the anniversary.
However, this year's demonstrations were banned and anyone participating faced being charged under the national security law with a maximum sentence of life in prison.
media captionSome of the celebrations at Tiananmen Square on Thursday