Hong Kong police warned on Sunday of safety risks associated with public assemblies, after they arrested 17 people and seized "significant quantities" of deadly explosives and chemicals that are believed to have been intended for attacking police officers at public events.

On the same day, a large number of radical protesters occupied roads and clashed with riot police in Tai Po in the New Territories. The protests were in opposition to the government's designation of clinics there to receive coronavirus patients.

In a press briefing on Sunday evening, the police said that during overnight operations across Hong Kong, they arrested a gang of 17 people involved in three homemade-bomb attacks in late January.

The 12 males and five females, aged 21 to 53, are believed to be responsible for three bomb attacks: in a bathroom at the Caritas Medical Centre on Jan 27; at the Shenzhen Bay checkpoint on Jan 28; and in a train at the Lo Wu checkpoint on Feb 2, said Steve Li Kwai-wah, senior superintendent of the Organised Crime and Triad Bureau.

During a search of about 22 locations across the city, police seized 2.6 metric tons of different chemicals from a unit of a commercial building in Tai Kok Tsui. All raw materials are believed to be those used to make powerful homemade explosives "commonly used by terrorists around the world", said Senior Bomb Disposal Officer Alick Bryce McWhirter.

In another unit of the same building, the police seized three nearly finished bombs, each containing 1.5 kilograms of explosives, and a batch of remote control devices used to trigger bombs. They are sophisticated electronics that use radio control as a triggering system, McWhirter said.

"We can say that it appears potentially a tragedy has been averted," McWhirter said. "It is clear that these bombers have no concern for the safety of others."

Li said the nearly finished bombs were intended for public activities to attack police officers.

He added that such homemade bombs and chemicals are rather unstable, and could explode indiscriminately at any time unintentionally.

Stressing that the bombs are deadly, Li said those who possess them are "extremely irresponsible".

Li said the police attach great importance to similar cases, and warned the public to be aware of such safety risks before attending any public activities.

The police have targeted other suspects, and will conduct further operations regarding the frequent bomb attacks and bomb hoaxes since January, he said.

Since late January, Hong Kong has seen at least four bomb attacks and 18 bomb hoaxes, especially at major transportation facilities, to protest the government's handling of the novel coronavirus outbreak.

No injuries were reported in those cases.