Seven Hong Kong police officers were in court Wednesday over the beating of a pro-democracy protester during mass rallies in 2014, an incident which was captured on film and beamed around the world.

On the first day of their highly anticipated trial, the seven men all pleaded not guilty to the attack on Civic Party activist Ken Tsang.

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 heavy-handed treatment of protesters during the rallies, which brought parts of the city to a standstill for more than two months. The demonstrators were asking for fully free elections for their future leaders.

Pro-police supporters gathered outside court, chanting "Love Hong Kong" and "We support our police force".

One lone pro-democracy protester set light to a photo of the officers, which was quickly doused by police.

The seven men, including senior officers, are charged with causing grievous bodily harm with intent on Tsang, while one also faces an allegation of common assault.

They were suspended from duty after their arrests.

The case is likely to centre around identification of those involved, whose faces were not completely clear in the dimly lit televised footage.

In a short first-day hearing, prosecution lawyers said five videos would be shown as evidence.

The defence have said they would challenge the authenticity of the videos.

Footage captured by local network TVB 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.

The accused officers sat stern-faced in court Wednesday, wearing suits and striped ties, speaking only to enter their pleas.

The hearing was cut short when lawyers said they needed time to study a new statement given by Tsang, who was not in court.

The trial will resume Thursday and is likely to take 20 days.

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

Tsang, who denied the charges, has said accusations against him were to distract attention from his case against the police.

The two incidents took place at the height of the mass protests.

Nearly 1,000 people were arrested during the rallies, including student pro-democracy leader Joshua Wong, who faces two protest-related verdicts in June.

The cases come as tensions remain high in the semi-autonomous city, with fears growing that Beijing is tightening its grip.

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.