Hong Kong's top court on Tuesday sent a teenager who shone a torch at police during a protest to a detention centre, overturning his earlier acquittal following an appeal.

In 2021, a magistrate found Mak Wing-wa guilty of taking part in an unlawful assembly with around 100 other people in Wong Tai Sin in October 2019, dismissing the defendant's claim that he had merely shone the torch back at officers who were shining their own lights at the crowd.

Last year, High Court judge Judianna Barnes quashed Mak's conviction, saying he may have used the torch just for fun and could have been acting independently of three other people next to him who had also shone things at officers.

The Department of Justice then took the case to the Court of Final Appeal (CFA) where a panel of five judges have ruled in its favour.

CFA judge Johnson Lam said Barnes had wrongly decided that Mak's actions were not connected to those of the other three who had shone the same torch and a laser pointer at police.

Given that Mak received the torch from a man at the protest, "it must be an irresistible inference" that he "intended to participate and took part in the unlawful assembly," Lam added.

The teenager will spend a maximum of six months in the detention centre.