A 70-year-old cleaner who worked for the Food and Environmental Hygiene Department died in a hospital on Thursday night after suffering severe head injuries during a conflict in Sheung Shui.

The initial investigation showed that the victim was hit by a brick while he was trying to make a video of rioters with his phone during his lunch break.

The Food and Environmental Hygiene Department expressed profound sadness at the passing of the outsourced cleansing service contractor worker, and is now providing necessary assistance to his family.

"We are very saddened by the incident and extend our deepest condolences to the family members of the deceased worker. We have requested the service contractor to follow up the labor insurance and compensation matters promptly," a spokesman for the FEHD said.

The Hong Kong Special Administrative Region government expresses outrage over the malicious acts of the rioters. The police will make every effort to investigate the case to bring offenders to justice.

Masked rioters have conducted extremely dangerous and violent acts in various districts, where they wantonly assaulted other members of the public, the spokesman said. The acts are outrageous, he added.

The spokesperson said police would take resolute law enforcement action to restore order, and the SAR government appealed to the public to remain calm and rational as well as to stay away from violence to safeguard their personal safety.

The victim is the first Hong Kong citizen to die in clashes between people with different political views since violent protests over the now-withdrawn extradition bill erupted in the city in June.

The man was among about 20 people who had been removing bricks left by rioters on Lung Wan Street in Sheung Shui around noon on Wednesday.

The victim was knocked unconscious after being hit on the head with a brick that is suspected of being thrown by a black-clad rioter. Two groups of people then started throwing bricks at each other. The rioters fled before police arrived. The case is under investigation.

Police have categorized the case as wounding with intent to do grievous bodily harm. In Hong Kong, offenders convicted of this offense face a maximum sentence of life imprisonment, according to the Offences against a Person Ordinance.

The elderly man is the second person to die in Hong Kong since protests began in June. The first was Chow Tsz-lok, 22, a student who died in hospital late last week after falling from a building.