A Hong Kong pro-democracy protester who was allegedly beaten by police in an attack captured by television cameras and beamed around the world stood trial on Monday over allegations he assaulted officers.

Political activist Ken Tsang stands accused of splashing liquid on police officers during mass street rallies in 2014 -- the same night as he was beaten in an attack.

The incidents took place at the height of the protests seeking free leadership elections in Hong Kong, rocking the reputation of the city's police force.

The police who allegedly beat him, who are not the same officers Tsang is accused of assaulting, are to stand trial separately.

The pro-democracy Civic Party member has pleaded not guilty to five charges -- two of assaulting a police officer and three of resisting a police officer.

On the opening day of the five-day trial, a courtroom packed with reporters and members of the public was shown video footage of officers pointing to a masked man who poured liquid onto the city's Lung Wo Road, close to government buildings.

In another piece of footage, taken from a local broadcaster, Tsang is shown being arrested.

Tsang, dressed in a suit, sat attentively on the defendant's bench. He donned a yellow ribbon-shaped badge, the symbol of the Umbrella Movement, named after the umbrellas used to ward off sun, rain, tear gas and pepper spray during the protests.

Outside court, the 40-year-old refused to say whether he would testify during the trial.

"I won't be able to say anything because of the legal procedures," he said.

It has taken a year for charges to be brought against both Tsang and the officers accused of assaulting him, further increasing controversy surrounding the case.

Tsang's lawyer, Robert Pang, criticised police reliance on news footage to identify Tsang.

"The identification took place after a year when someone showed a video with which he was identified... it is inappropriate," he told the court.

Tsang has said police brought assault charges against him to distract from the case against them.

Video footage aired by local television network TVB at the time showed a group of men hauling a handcuffed Tsang to a dark corner in a public park in the early hours of October 15, 2014. One stands over him and punches him, as three others are seen repeatedly kicking him.

Seven policemen will stand trial for the assault on the activist later this year. They have pleaded not guilty to the charges.

Tsang has presented his case to a UN Committee Against Torture in Geneva.

Thousands of protesters blocked major thoroughfares in Hong Kong in 2014, calling for a free vote on the city's chief executive, but the protests failed to win any concessions from either Beijing or the city authorities.