“It must be recognised that this was an extremely testing time for a generation of police officers who had never had to face such large-scale manifestations of civil disorder and disobedience, in which they would inevitably, and no doubt reluctantly, become embroiled,” Macrae wrote.

“Whatever the pressure and stress the appellants were under, and however great the insult and provocation they perceived from participants in the events that night, there was no excuse for what they did to [the victim].”

But the judges unanimously quashed the convictions of Constable Lau Hing-pui, 42, and Detective Constable Wong Wai-ho, 40, because neither was identified by witnesses in video recordings or photographs to prove they were part of the group. That being the case, they said, their appeals “must be allowed”.

The five officers who would return to complete prison sentences were Chief Inspector Wong Cho-shing, 52; Senior Inspector Lau Cheuk-ngai, 33; Detective Sergeant Pak Wing-bun, 46; and detective constables Chan Siu-tan, 35 and Kwan Ka-ho, 35.

The judges concluded the officers’ starting point sentence at 30 months “may be said to be manifestly excessive” and further disagreed with the lower court’s finding that all of the participants in this joint enterprise were of equal culpability.

The sentences of the remaining five officers, who failed to overturn their convictions, were reduced to between 15 and 18 months.

Lau and Wong briefly patted their co-defendants’ shoulders before walking out of the courtroom.

But neither responded to questions when asked how they were feeling or whether they would remain on the force.

Among the small crowd of police supporters in the public gallery was a Mandarin-speaking woman who wailed upon learning that some officers would return to jail.

“We couldn’t help you,” she said between sobs. “Hong Kong’s rule of law is dead.”

She had to be escorted out of the courtroom after her body went limp in her friends’ arms.

‘I was in great pain,’ Ken Tsang tells Hong Kong court in police assault trial

News footage of Tsang curling up against the officers’ kicks, punches and baton strikes quickly circulated in 2014, making it one of the most controversial scenes from the largely peaceful protests, while the site gained infamy as the “dark corner”.

Subsequent medical examinations found the social worker sustained swelling and reddish bruises on his face, neck, shoulder, flank, chest and back.

Tsang said on Friday he could not accept the ruling, which he believed would send a message to the police that prosecutions for such offences are difficult even when supported by photos and video evidence.

“Like most Hongkongers I feel angry and hopeless,” Tsang said. “I expect the police to become even more unruly.”

He added that he hoped prosecutors would take the case to the Court of Final Appeal.

A spokesman for the Department of Justice said prosecutors would study the judgment and case report to determine whether any follow-up action is called for.

The police force declined to comment on the case, given the possibility of further appeals.

District judge David Dufton jailed all seven officers for two years, after granting a six-month discount to factor the circumstances of the Occupy protests as well as personal factors such as the officers’ clear records, service to the community and likelihood of losing their pensions.

Chan alone was sentenced to one more month on one count of common assault for twice slapping Tsang at a police station, to be served concurrently with the longer jail term.

He dropped his appeal and served time the following year, after his assailants were jailed.