The national security law was introduced by Beijing in 2020 to clamp down on protesters.
Five teenagers have been sentenced to three years’ detention in Hong Kong for advocating overthrow of the Beijing government.
It is the first time the national security law has been used in court against under-18s in Hong Kong.
Beijing introduced the wide-ranging law – which made it easier to prosecute protesters – in the city in 2020.
Many who defy the Chinese government have since been jailed, removing much of the political opposition.
The court heard the defendants had used social media and street booths to advocate a “bloody revolution” to overthrow the Chinese state in the former British colony.
Judge Kwok Wai-kin said: “Even if one person is incited, Hong Kong’s stability and residents’ safety could have been greatly harmed.”
The teenagers – aged between 16 and 19 – were members of Returning Valiant, a pro-Hong Kong independence group.
Mr Wai-kin said he appreciated the defendants’ “age and immaturity”, which meant they were sentenced to a detention facility for young people – also known as a training centre – instead of going to prison.
The judge also capped the length of their sentence to three years. How long they remain in custody will remain at the discretion of authorities.
The case also involves two adults, who will be sentenced next month.