A Hong Kong court on Friday jailed two young men in relation to a violent clash during the city's political unrest in 2019 that killed an elderly man.

Luo Chang-qing, a 70-year-old cleaner, died from a blow to the head during a border town confrontation where democracy protesters and government loyalists hurled bricks at each other.

Luo was one of the only fatalities directly linked to Hong Kong's political turmoil in 2019 and his case remains a source of outrage for government supporters.

Kelvin Lau, 19, and Chan Yin-ting, 18, were each sentenced to five-and-a-half years in jail for rioting on Friday.

The duo faced additional manslaughter and wounding charges but a jury last month cleared them of responsibility in Luo's death.

The judge said on Friday that the defendants - who were underage at the time of the crime - were among the group that instigated the violence that escalated into a "very serious riot".

"Things devolved into mayhem which led to very tragic consequences," the judge said.

The skirmish, which took place in the Sheung Shui neighbourhood in November 2019, was one among dozens that broke out between ideological opponents at the height of Hong Kong's massive pro-democracy protests.

The defendants made plans to protest together on the day, which showed some level of premeditation, according to the judge.

Earlier media accounts of the Sheung Shui conflict placed blame on both sides, but the judge praised the government loyalists as "unsung heroes" who acted selflessly and courageously when they stood up to the protesters.

The judge said a deterrence sentence was needed, with a minor reduction after considering the duo's young age and their lack of adult supervision.

The judge also commended the police for being "Asia's finest" and conducting a thorough investigation into the case.

At trial, defence lawyers argued that the duo were threatened by police to admit responsibility for the death.

In court, police officers denied allegations of coercion or intimidation.