Seven Hong Kong police officers were jailed for two years Friday for what the judge described as a "vicious" attack on a protester during pro-democracy rallies in 2014, captured on film and beamed around the world.

All seven were convicted earlier this week of assault causing actual bodily harm to Civic Party activist Ken Tsang, but were acquitted of a more serious charge of grievous bodily harm with intent.

Video footage of the attack, filmed by a local network near the city's government headquarters, shocked residents and dented their faith in the usually trusted police force.

It showed a group of men hauling a handcuffed Tsang to a dark corner in a public park, where he was beaten.

One man stood over him inflicting blows while others were seen repeatedly kicking him.

It was "a vicious assault", said judge David Dufton as he handed down sentencing at the city's district court, adding that police officers who break the law "have to be made an example of".

The maximum sentence for the lesser assault charges was three years.

"The multiplicity of the injuries and the damage to Hong Kong's reputation... make this a very serious case," said Dufton, noting the incident had made international headlines.

Police have been criticised for their sometimes heavy-handed treatment of protesters during the 79 days of rallies and street blockades, known as the Umbrella Movement, that brought parts of the city to a standstill.

The demonstrators were seeking fully free elections for semi-autonomous Hong Kong's future leaders.

The rallies were sparked after Beijing said the public could choose the next chief executive, but only after candidates had been vetted.

There are increasing concerns in Hong Kong that Chinese authorities are tightening their grip on the city.

Social worker Tsang, who was not in court Friday, said the sentencing proved that ordinary people can "fight back" against police violence.

"I hope the government and the police will do something, think about what they have done, and say sorry to Hong Kong citizens," he told AFP.

- 'Human frailty' -

The court found that one officer had stamped on Tsang and hit him with a baton.

Four other officers kicked him while two other officers did not participate but watched.

One of the officers who kicked Tsang was also found guilty of common assault for slapping him in the face twice after he was taken to a police station.

Dufton said that Tsang, now 41, was "most fortunate" not to have received more serious injuries from the attack.

Pro-democracy campaigners gathered outside the court said they thought the sentencing was fair.

But incensed pro-police demonstrators said the officers had been unduly punished and that they would march in protest Saturday.

"We want society and the judge to see how angry we are," said Tang Tak-shing, chairman of a police support group.

The defence argued in mitigation that the officers had been under intense pressure during the protests, which they said had dented morale, with a number of police injured.

Defence lawyer Lawrence Lok attributed the assault to "human frailty".

Political analyst Joseph Chen said the sentencing showed the city's legal system was still intact.

"There is ample evidence that we still have an independent judiciary," he told AFP.

Tsang was himself found guilty last year of assaulting and resisting officers on the same night, when he splashed an unspecified liquid on police.

He was given a five-week sentence and has said he will appeal.