The court of final appeal listened to arguments on the use of the "joint enterprise" doctrine that allows prosecutors to indict those not physically present at a riot or unlawful assembly with the same charges as actual participants in a hearing yesterday.

The appeal was initiated by Tong Wai-hung, who was among the three defendants who were acquitted of rioting charges in Sheung Wan in July 2019 by the district court.

The court of final appeal also granted leave to appeal to Lo Kin-man, who was sentenced in 2018 to seven years in prison after being convicted of rioting for taking part in the Mong Kok clashes in 2016, so he could argue against the "joint enterprise" doctrine. While the final verdict of the case would not affect the acquittals of Tong's cases, it might affect riot and unlawful assembly cases in the future.

The Department of Justice's Anthony Chau Tin-hang said the "joint enterprise" doctrine in common law should be applicable to all offenses under the common law with no exception, or it might lead to a big legal loophole.

He also said if the principle - that a defendant's presence at the scene is not always necessary for criminal liability under "joint enterprise" - becomes inapplicable, it will be unable to convict those who are involved in the cases.

This is because masterminds, or those who abet riots may not always be present at the scene, and that there have been cases where such people have instead used communications like Telegram, he said.

But senior counsel Gladys Li Chi-hei, who represents Lo, said earlier that if the principle is applicable to the "joint enterprise" doctrine, it would go against the accusations of unlawful assembly, as the prosecution had to prove what kinds of acts the accused had done to disrupt social peace, a point that is relevant to those at the scene.

She also said the prosecution must identify the initial members with a common purpose of committing a breach of the peace in an unlawful assembly, before putting charges for unlawful assembly or riot to avoid abusing the charge against those who were accidentally at the scene and committed violent acts.

Senior counsel Philip Dykes, for Tong, also said that law enforcement would issue warnings before arresting those who have violated the riot offense, indicating that the offense is not applicable to those who are not present at the scene.

No dates have been set for the release of the final ruling.