Friday’s ruling marked the first time an appellate court quashed a not guilty verdict recorded in a riot trial since the Court of Final Appeal outlined the criteria for proving the offence in a landmark ruling in late 2021.

The judgment made it clear prosecutors were required to prove a suspect’s participation, or “participatory intent”, before proceeding with a case of taking part in a riot or unlawful assembly.

But defendants can still be convicted even if they have not been seen acting violently in demonstrations if the court is satisfied they have encouraged others to join an illegal gathering through their attire, the protective equipment in their possession, or a deliberate act to remain at the scene of unrest.

The 13 accused were acquitted in two separate trials overseen by Judge Sham Siu-man, who has since retired, before the top court’s ruling.

In one case, Sham cleared five men and two women of rioting by pointing to a lack of evidence of what they had done before their arrests in Wan Chai on August 31, 2019.

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The judge further ruled prosecutors had failed to establish a prima facie case against the eighth accused, social worker Jackie Chen Hung-sau, who spoke to police through a microphone and called for officers to calm down.

But the High Court’s Chief Judge Jeremy Poon Shiu-chor said Sham had overlooked the fact that the accused could “facilitate, assist or encourage those taking part in a riot” by wearing outfits similar to the offenders.

Poon, joined by justices Derek Pang Wai-cheong and Anthea Pang Po-kam on the bench, also accused Sham of being sympathetic to those arrested during the 2019 protests.

He said it was pure speculation for Sham to suggest some of the accused might have merely wished to witness “a rare and special historical moment”.

“Calling a riot a ‘rare and special historical moment’ is utterly inappropriate,” the chief judge said. “That is not to mention a riot is not an occasion that calls for observers.”

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Sham’s notion that some people might have covered their faces to avoid being confused with the rioters was “far-fetched” and “completely detached from reality”, Poon said.

Sham had also wrongly decided that prosecutors lacked evidence to charge social worker Chen, the chief judge said, adding the defendant could have encouraged riot participants to perpetuate the disturbance with her physical presence.

In the second case, Sham cleared three men and two women of conspiracy to take part in a riot by highlighting the lack of proof they had intended to use the dangerous items found in a Wan Chai flat to cause a disturbance during a protest in the neighbourhood on October 1, 2019.

Sham said the group was arrested before the demonstration and should not be convicted in hindsight just because of what happened on the day concerned.

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The chief judge was not convinced by that reasoning, however, saying the unrest on October 1 was compelling evidence that Sham should have taken into account.

Chen and three other co-defendants in her case are now required to stand a retrial before another judge. The appeal court also rescinded Sham’s ruling for prosecutors to bear Chen’s legal expenses in the trial.

Poon stressed the remaining nine, including the five facing the conspiracy charge, would have been tried again had they not left Hong Kong before the prosecutors appealed.