Seven Hong Kong police officers were found guilty Tuesday of assaulting a protester during pro-democracy rallies in 2014, in an attack that was captured on film and beamed around the world.

All seven were convicted 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 fore.

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.

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

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

Hong Kong's district court found that one officer had stamped on 41-year-old Tsang and hit him with a baton, causing circular reddish bruises.

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

"Every police officer has a duty to prevent the commission of a crime, even by fellow police officers," judge David Dufton said in a press summary of the verdict.

It added that Tsang suffered injuries to his face, neck and body during the assault.

"The court was not however satisfied these injuries amounted to grievous bodily harm but was satisfied they amounted to actual bodily harm," the press statement said.

The charge carries a maximum sentence of three years.

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.

- 'Human frailty' -

Following the verdict, a lawyer for the defence said in mitigation that the protests had dented police morale, with a number of officers injured.

"Human frailty resulted in the transgression of seven devoted police officers," Lawrence Lok told the court.

He said defendant Wong Cho-shing, one of those who watched the attack, had worked long hours during the protests and faced physical and verbal abuse from demonstrators.

Mitigation continued Tuesday afternoon and a sentencing hearing was set for Friday. The officers will be remanded in custody in the meantime.

Tsang was not in court for Tuesday's verdict and told AFP he won't comment on the case until after Friday's sentencing.

The social worker 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.

He has always argued that police brought assault charges against him to distract from the case against them.

Rival protesters gathered outside the court Tuesday, with pro-police supporters outnumbering a small group of democracy demonstrators.

Using loudspeakers they chanted: "Support the seven officers" and "Reasonable enforcement of the law".

Nearly 1,000 people were arrested over the course of the 2014 protests.

Rally leaders Nathan Law, Joshua Wong and Alex Chow were all convicted in August for taking part in, or inciting others to take part in, an unlawful protest that led up to the major demonstrations.

They were given community service or suspended sentences after the magistrate said she believed they had been "genuinely expressing their views" during the protest, which saw students climb over a fence into the Hong Kong government complex.