Hong Kong Chief Executive Carrie Lam Cheng Yuet-ngor sounded an alarm on Tuesday over real threats to national security in the city, as Hong Kong police announced the arrests of members of a radical group who planned to detonate homemade bombs this month in tunnels, court buildings and trash cans.

Lam said recent sporadic cases of violence showed that the violence that had traumatized the special administrative region in 2019 has shifted from collective, street-based activities to personal, underground ones, posing a real and present danger to the city.

Lam's remarks came about an hour before Hong Kong police announced the arrests of nine people between the ages of 15 and 39, all from a radical separatist group, over a bomb plot.

The case followed the attempted murder of a police officer by a "lone-wolf terrorist" in the business district on Thursday and the arrest of two other people in connection with an arson attack on Government House－the chief executive's residence.

Lam said the attempted murder of the police officer should alarm Hong Kong society, and she called for vigilance against distorted information, violence and incitement.

"The national security threats have come from external, anti-China forces, coupled with fugitives who fled the city during and after the 2019 social unrest", Lam said. They have wantonly denigrated the city's efforts to safeguard national security and stirred up discontent among the people of Hong Kong.

They were aided by unregulated social media platforms that are awash with disinformation, glorification of violence, and hatred against the central government, the HKSAR government and its police force, she said.

Such incitement has weakened the sense of compliance with the law among some local residents, she added.

The nine suspects, who include six secondary school students and faculty members of a university and a secondary school, were arrested under the National Security Law for Hong Kong by the police force's national security unit.

Some group members hid in a hostel room in one of the city's busiest business areas, Tsim Sha Tsui, to make powerful bombs similar to those used in terrorist attacks overseas, said Li Kwai-wah, senior superintendent of the Police National Security Department.

Speaking at a news conference, Li said the seizure of documents and brochures unveiled a concrete and well-organized plan and shocked police officers.

According to a preliminary investigation, the group planned to instigate secondary students this month to carry out bomb attacks at public places, including the Cross-Harbour Tunnel, courtrooms and railway stations, and then send them away from the city.

Police also seized explosive substances, bomb-making equipment, communication equipment, air rifles and HK$80,000 ($10,300) in cash. They also froze a total of HK$600,000 in related funds.

The city's Secretary for Security Chris Tang Ping-keung said on Tuesday he was furious that someone used secondary students as pawns to implement their plan and to help criminals evade punishment.

He also told reporters that some people continue to incite innocent residents to support or sympathize with these "terrorists".

He warned that those who sympathize with violent attackers might participate in such violence in the future.

At a session of the Panel on Security of the Legislative Council, the Liberal Party's Peter Shiu Ka-fai criticized some law scholars and netizens in the city for downplaying the July 1 knife attack on a police officer. A 50-year-old man plunged a knife into the back of an on-duty Police Tactical Unit officer and then killed himself. The officer, age 28, received a 10-cm-deep wound and one of his lungs was pierced.

Meanwhile, on Tuesday, Lam also announced the appointment of seven members of a newly established committee that will review and confirm the election eligibility of candidates for chief executive, the Legislative Council and the Election Committee responsible for selecting the city's top leader.

Chief Secretary for Administration John Lee Ka-chiu was appointed as chairman of the Candidate Eligibility Review Committee.

The other members of the committee include Chris Tang, former secretary for justice Elsie Leung Oi-sie and former Legislative Council president Rita Fan Hsu Lai-tai.