Chief Superintendent Au Chin-chau, head of the force's Organised Crime and Triad Bureau, said: "There are possibilities that this group of people may want to do something in those particular locations."

READ MORE: Hong Kong bomb plot a conspiracy to smear us, localists claim, ahead of reform vote

Sources told the police had monitored the group for more than a month and had learned of previous testing activity at the former ATV film studios in Ho Chung in Sai Kung, where the first batch of explosives was seized. The sources said the force was not describing the bomb plot as a terror attack as they had yet to identify a specific target - and the terror threat level remained "moderate".

The plot was smashed late on Sunday night when two men were detained while trying to test explosives at the former studios. Several kilograms of suspected explosives in solid form and two litres of suspected liquid explosives were seized there.

The explosive seized in the raids was triacetone triperoxide - also known as TATP - which has been used in deadly terrorist attacks around the world, including in Israel and the London bombings on July 7, 2005, in which 52 people died and more than 700 were injured.

"Initial examination showed it was a mixture of chemicals and the major ingredient used for making TATP. It was ready to use," one police source said.

Police were last night investigating the scale of devastation that the materials could have wreaked, but Au said a small quantity of TATP was powerful enough to blow a car to pieces.

On the two men's suspicious activities at the defunct studio, Au said: "Police investigations revealed that the two men planned to test the powder of an explosive device at the location in Ho Chung."

Another two litres of a raw material that is a main ingredient in TATP was also seized at the Sai Kung residence of one of the two suspects. Police also seized the other suspect's mobile phone that allegedly stored the formula for making smoke grenades.

In a series of subsequent raids, officers arrested seven others across the city and seized several air guns, shotguns, bottles of paint thinner, Guy Fawkes masks, a tablet computer and mobile phones.

According to police, the tablet computer stored the sketch of the explosive device and a formula for making a smoke grenade.

The suspects include a female tertiary student, a teaching assistant, a construction worker, a technician and three unemployed people. It is not known what the others do.

The tenth person, a 58-year-old man, was arrested trying to leave Hong Kong at the Lo Wu border checkpoint. He is a businessman who works on the mainland and is believed to be the father of one of the suspects, a police source said.

All of the suspects were arrested on suspicion of conspiracy to manufacture explosives, an offence that carries a maximum penalty of 20 years in prison.

Two of them have had brushes with the law. They were among 38 people arrested in a clash between protesters against cross-border traders and their opponents in Yuen Long in March, according to another police source.

Yesterday, Au reiterated that "police would not tolerate any act affecting public order and public safety and will take resolute and effective action to restore social order".

A five-day rally outside the Legco complex in Admiralty against the government's reform package, organised by the Civil Human Rights Front, began on Sunday and continued last night.

Meanwhile, security will be tightened at the Legco building from tomorrow, when the government's plan for the 2017 chief executive election will be tabled for debate.

Educational tours will be suspended, the lawmakers' cafe will close, no guests will be allowed and no more than five assistants will be allowed to work in the office of each lawmaker.

Lawmakers are expected to vote on the package on Friday.

EXPLOSIVE CAN BE CREATED WITH D.I.Y. EASE

A highly unstable explosive called triacetone triperoxide, or TATP, appears to be the main weapon of destruction that a local radical group allegedly planned to unleash on Hong Kong - and is the same type of bomb used in major terror attacks elsewhere.

TATP was not easily available in a finished state, security risk consultant and former head of the police's criminal intelligence team Steve Vickers said.

"But the internet and YouTube make it easy to come up with the ingredients - the persons who are accused of possessing this [unless they are experts] probably present a greater threat to themselves than the public if they try and transport this material," Vickers said. He added that without any chemical analysis reports on the hazardous discovery, it would be hard to make a full assessment of the threat it posed.

Fung Ying-sing, associate professor of the University of Hong Kong's chemistry department, said raw materials for making TATP, such as hydrogen peroxide, were easily found in pharmacies and fertiliser shops.

And according to Britain's Royal Society of Chemistry website, common household ingredients - paint thinner (acetone), bleach or antiseptic (hydrogen peroxide), and a powerful drain unblocker - could be mixed in the right proportions to produce white TATP crystals. The society warned, however, amateurs risked hurting themselves in the process.

In October, a teen tried to light a test tube full of TATP powder at Aberdeen police station while telling officers he wanted to die. Officers subdued him and called in the bomb squad. In March 2009, three teens were arrested for possessing TATP. One of the boys, aged 13, set off a home-made bomb at his flat in Kwun Tong.

Little-known group advocates ‘revolutionary activities’

"The National Independent Party" was an unknown group operating below the radar until yesterday, when police uncovered an alleged bomb plot by its core members.

Prior to the discovery, the name of the Hong Kong pro-independence group, which was set up in January according to its Facebook page, had never appeared in any media, according to searches done by the .

In a "warning" posted on June 1, the group said: "If the electoral reform is passed on Wednesday, Hongkongers have to be mentally prepared there will be casualties on that day; the Legislative Council will come to ruin like in Ukraine."

In yet another post, on April 13, the group pledged to "conduct revolutionary activities anonymously in the future". It said it would target the Legco electoral reform debate this week and the third airport runway, and would "secretly pull them down by all means".

A video clip showing protesters throwing objects in a clash with riot police abroad was accompanied by the message: "This is the standard of a struggle. Every warrior always has to stand ready."

As for clues on its ideology, a 700-word article dated May 12 states: "We are devoted to connecting pro-independent groups in Taiwan and Hong Kong to form a new cross-boundary independent power. And our party has been working on this key issue behind the scenes."

It urged Hongkongers not to adopt the definition of "non-violent struggle" as advocated by some local politicians.

Instead, they should take a leaf from the experience of Taiwan's Democratic Progressive Party as a "standard" for non-violent struggle, where the use of petrol bombs and tyres in forming road barriers did change the political landscape on the island.

"The battle for Hong Kong in the future will be like how the Democratic Progressive Party confronted the Kuomintang," the post read.

The group said members had taken part in protests against cross-border traders - a series of such protests turned unruly in February.

"Liberty, not communism, is the most contagious force in the world" - a quote from former United States chief justice Earl Warren - appeared at the top of the Facebook page, which had more than 400 "likes" before it became unsearchable late last night.

Danny Mok