Hong Kong police are planning large-scale anti-terrorism drills and will consider invoking international anti-terrorist legislation for the first time when prosecuting suspects in recent bomb-making cases linked to anti-government protests.

While they would be unable to build a terrorism case without the government legally categorising individuals or groups as terrorists - which is highly unlikely at this stage - the move would be a symbolic one to highlight how serious the situation has become, a source said.

That came as police arrested 17 people on Sunday and seized 2.6 tonnes of explosives in raids related to three earlier bomb plots at public facilities, and warned of the growth of local terrorism.

Under the city's United Nations (Anti-Terrorism Measures) Ordinance, Hong Kong's leader could apply to the court for an order to specify that a suspect was a terrorist or terrorist associate.

Senior superintendent Steve Li (centre) speaks at a briefing after Sunday's arrests. Photo: Felix Wong

The maximum sentence for causing an explosion of a nature intended to endanger life under the legislation is life imprisonment, the same penalty as for explosives-related offences under the Crimes Ordinance, the law normally used in such cases.

The police source said invoking the anti-terrorism law to bring suspects to justice would have a more "symbolic meaning" despite the similar jail terms, as the force had foiled at least 10 bomb plots since the anti-government protests broke out in June last year.

"It could truly reflect the illicit acts and the current situation faced in Hong Kong, although the suspects risk the same jail terms under the Crimes Ordinance," the source said.

The ordinance, which has never been used, was enacted in Hong Kong in 2002 in line with a UN resolution after the September 11 attacks in the United States and spells out terrorism-related offences.

In a press conference on Monday, Assistant Commissioner of Police Albert Yuen Yuk-kin was asked whether the force would consider using the legislation to deal with the recent arrests.

Yuen said: "We will consider the evidence available in each individual case and maintain close communication with Department of Justice. We do not rule out any possibility."

He declined to say whether there was an immediate need to raise the city's threat level from "moderate" to "high", meaning the possibility of an attack and specific intelligence suggesting that Hong Kong was likely to be a target.

But he said the force would conduct large-scale drills with other government departments and map out policies to raise readiness for home-grown terrorism, and the force would continue to gather intelligence and assess the risks.

Early on Sunday, officers raided 22 locations and found three bombs, each containing 1.5kg of explosives, and three remote-controlled devices.

Some 2.6 tonnes of chemicals - including black powder, nitrate mixture, sulphur, magnesium powder, and sodium - were found in one flat, along with six bottles of drain cleaner and 60 electric matches.

Police believed they were linked to three bomb plots in January and February, when an explosive device went off at Caritas Medical Centre in Cheung Sha Wan, a day before another one was found at Shenzhen Bay control point. At Lo Wu railway station, one of two devices found on a train also caught fire and spurted white smoke.

Senior Superintendent Steve Li Kwai-wah, deputy head of the organised crime and triad bureau, said more explosives, including pipe bombs, were seized as evidence after Sunday's arrests.

He said the bombs earlier placed at the public facilities were intended to not only hurt police officers but also civilians. Li raised concern over the escalating violence and public safety.

"We understand people can have their own beliefs or agenda. But I believe no civilised place would accept people using home-made bombs to threaten the government or citizens to follow their views," he said.

I believe no civilised place would accept people using home-made bombs to threaten the government or citizens to follow their viewsSteve Li, senior superintendent

Some people had claimed responsibility on a social media platform before and after the plots, saying they intended to pressure the government into closing the city's border with mainland China in response to the deadly coronavirus.

In an official statement released last Monday to reveal the city's crime situation in 2019, the force labelled certain violent acts driven by the protests as "home-grown terrorism" and slammed online materials that were in effect "weapons-making handbooks" or "guides" to kill police officers.

The UN ordinance defines a "terrorist act" as an action that causes serious violence against a person, serious damage to property, endangers a person's life, creates a serious risk to the health or safety of the public, seriously interferes an electronic system, or seriously disrupts an essential service, facility or system.

It also categorises terrorism as actions or threats intended to compel the government or an international organisation, or to intimidate the public, for the purpose of advancing a political, religious or ideological cause.

A "terrorist" is defined as a person who commits, or attempts to commit a terrorist act.

The chief executive could make an application to the Court of First Instance for an order to specify a person as a terrorist or terrorist associate.

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