Nine people, including six secondary school pupils, have been arrested in Hong Kong on suspicion of engaging in terrorist activity, with police claiming to have uncovered an attempt to make explosives and plant bombs across the city.

The group of pupils were attempting to make the explosive triacetone triperoxide (TATP) in a homemade laboratory in a hostel, police said.

They planned to use TATP to bomb courts as well as cross-harbour tunnels and railways, and even planned to put some of these explosives in rubbish bins on the street “to maximise damage caused to society”, police said.

The nine arrested are five men and four women aged between 15 and 39, said senior superintendent Li Kwai-wah of the Hong Kong Police National Security Department.

Authorities said they seized apparatus and raw materials used to make the TATP, as well as a “trace amount” of the explosive.

Police also froze about 600,000 Hong Kong dollars (£55,000) in assets that they say may be linked to the plot.

The group all planned to leave Hong Kong for good after the sabotage, authorities said.

The arrests come two years after months of massive anti-government protests rocked the city.

TATP has been used in terrorist attacks worldwide. Since 2019, Hong Kong police have arrested several people over alleged bomb plots and for making TATP.

Hong Kong chief executive Carrie Lam said at a regular news briefing on Tuesday that she hopes the public will “openly condemn threats of violence”.

“They should not be wrongly influenced by the idea that there is only government tyranny … but that breaking the law is in order, if you’re trying to achieve a certain cause,” she said.

“They should not be influenced into thinking that they can find excuses to inflict violence.”

Ms Lam said that an envelope of “white powder” had been sent to her office.

Police said Tuesday that the substance was still being analysed but that they did not believe it to be dangerous.

In December 2019, authorities defused two bombs at a local Catholic school.

A remote-controlled homemade bomb was also detonated near a police car in 2019, when the anti-government protests were ongoing.

Last week, a Hong Kong man stabbed a police officer with a knife, before killing himself.