Hong Kong police on Friday classified the death of an elderly man who had sustained a critical head injury during a clash with protesters as murder.

The 70-year-old man was hit in the head with a brick when he was clearing bricks left by protesters on a street near the North District Town Hall in Hong Kong's Sheung Shui on Wednesday. He was treated at the Prince of Wales Hospital, but later succumbed to his injuries late Thursday night. 

The man, who worked for Hong Kong's Food and Environmental Hygiene Department (FEHD), is the first non-protester people killed amid the unrest in Hong Kong.

He was on his lunch break when he was hit in the head, the police said. 

The Hong Kong Special Administrative Region (HKSAR) government has expressed outrage over the misdeeds of violent protesters and said the police will bring the perpetrators to justice.

The FEHD, in a statement on Friday, expressed its condolences and said the organization is working to provide assistance to the relatives of the deceased.

The investigation is ongoing, but the police have suspects in custody, said Kong Wing-cheung, senior superintendent of the Police Public Relations Branch.

(CGTN reporter He Weiwei contributed to the story)