A Hong Kong man has been sentenced to six years in prison after pleading guilty to a terrorism charge under a Beijing-imposed national security law over his involvement in a foiled plot to bomb court buildings.

Prosecutors said Ho Yu-wang, 19, was the plot mastermind who planned to manufacture explosives and target court buildings in 2021.

The plot, involving mostly secondary students at the time, was foiled due to a police investigation, while no bombs were made and no casualties occurred, the prosecution said.

Police said they raided a guesthouse room in 2021 and seized equipment believed to be used for making explosives.

They also alleged Ho had written notes saying that his goal was to destabilise Hong Kong, promote conflicts between the central government and others, and build up a resistance group.

Two other defendants received a jail term of up to six years for an alternative charge.

Ho is a lesser-known activist in the semi-autonomous city’s pro-democracy movement, but his case has drawn attention because most of those arrested for the plot were students when the prosecution began about two years ago.

In May, Ho pleaded guilty to conspiracy to organise, plan or commit terrorist activities under the security law imposed on the former British colony following the 2019 protests.

The security law enacted in 2020 criminalizes acts of succession, subversion, terrorism and collusion with foreign forces, leading to the prosecution of many leading activists in the city. Beijing and Hong Kong authorities hailed it for bringing back stability to the financial hub.

Judge Alex Lee said if the plot had been carried out it would have changed the social conditions in Hong Kong from bad to worse, and Ho had disregarded the rule of law and the risks of his “fellow gangsters”.

He reduced his term from a starting point of 10 years, based on grounds that Ho made a timely plea and provided practical assistance to the police later.

In his mitigation, Judge Lee heard that Ho was grateful he had been arrested, blocking his plan from materialising, and that he has changed his mindset and resumed his studies, particularly of Chinese history.

Two other defendants – Kwok Man-hei, 21, and Cheung Ho-yeung, 23 – were sentenced to two-and-a-half years and six years in jail respectively. They pleaded guilty to conspiring to cause explosions that were likely to endanger life and property, an alternative to the terrorism charge that falls under a separate law.

In May, four people involved in the plot were already sentenced to jail or rehabilitation-focused training centres following their guilty plea of the alternative charge.