A Hong Kong protester told Wednesday how he was kicked and punched during pro-democracy rallies in 2014 as seven police officers stood trial over the attack, which was captured on film and beamed around the world.

Civic Party activist Ken Tsang testified for the first time in the trial which began last week in the semi-autonomous Chinese city.

All seven officers have pleaded not guilty.

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

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

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

Tsang, 40, said he was assaulted by seven men after he was subdued by police at a rally.

He was shown video footage in which prosecution lawyers asked him to identify himself. In one clip, he was thrown to the floor before several men kicked him in a dimly-lit area.

Footage by local network TVB and broadcast at the time 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 punching him while three others were seen repeatedly kicking him.

Defence lawyers have said they would challenge the authenticity of the videos.

One of them told the court there were inconsistencies in what Tsang had told police in his early statements, including his failure to point out the exact number of people who assaulted him.

Cheng Huan suggested Tsang had no "independent memory" of what happened and was influenced by footage that became widely available afterwards.

The seven police, including senior officers, are charged with causing grievous bodily harm to Tsang, while one is accused of common assault.

They were suspended from duty after their arrests.

Tsang himself was sentenced to five weeks in prison for assaulting and resisting officers after he splashed liquid on police on the same night he was beaten.

Pro-police supporters gathered outside court, chanting "We support our police force" while several Tsang supporters carried yellow umbrellas, a symbol of the democratic movement.

Nearly 1,000 people were arrested over the course of the protests, with student leader Joshua Wong facing a verdict later this month.

He was acquitted Tuesday in another protest-related case.

The hearings come as tensions remain high in the city.

Hong Kong was returned to China in 1997 under a "one country, two systems" agreement and enjoys much greater freedoms than in mainland China, but there are concerns those freedoms are being eroded.