Ho Yu-wang, 20, Kwok Man-hei, 21, and Cheung Ho-yeung, 23, were all charged with "conspiracy to commit terrorism" under the National Security Law.

High Court Judge Alex Lee said he accepted that "the hostile social atmosphere" during Hong Kong's protracted pro-democracy protests could "easily cloud one person's moral judgment... (and) might turn people with previously good characters into radicals".

Kwok belonged to a pro-independence group named "Returning Valiant", according to the prosecution's summary of facts.

The three were accused of planning to make improvised explosives devices and place them in public places including government offices, cross harbour tunnels, police staff quarters, railways and court buildings between April 1 and 5 July 2021. The defendants were arrested before any of the devices were made or used.

Ho, who was alleged to be the mastermind behind the plan, and Cheung were sentenced to six years imprisonment, while Kwok was sentenced to 30 months imprisonment.

Ho pleaded guilty to the terrorism charge while Cheung and Kwok pleaded guilty to an alternative charge of conspiring to "cause explosions likely to endanger life or to cause serious injury to property."

The starting point of Ho's sentence was set at 10 years and Judge Lee granted him four years deduction for his assistance to the prosecution that led to a guilty plea from Cheung.

Four others involved in the plans were sentenced in May. One 21-year-old was jailed for five years and eight months for renting a flat for the scheme, while three others aged between 17-20 were sent to training centres.

Beijing imposed a national security law on Hong Kong in 2020 to punish secession, subversion, terrorism, collusion with foreign forces with up to life imprisonment.

Some foreign governments including the U.S. say the security law is a tool to crush dissent. Beijing says the security law has restored stability to the global financial hub after the mass 2019 protests.

Since the national security law was imposed, more than 280 people have been arrested including opposition politicians, activists, lawyers and journalists.

(Reporting by Jessie Pang and Edward Cho; Editing by James Pomfret and Lincoln Feast.)

By Jessie Pang and Edward Cho