A third officer, seen in plain clothes in the footage and who did not report the case, was arrested in the evening on a conspiracy charge.

By Tuesday night, none of the trio had been charged.

In eight minutes of security camera footage from the June 26 incident – which was not related to the protest movement sweeping the city – two policemen are seen repeatedly hitting their 62-year-old male victim in the head, abdomen and genitals, in North District Hospital, Sheung Shui.

The victim, surnamed Chung, had been arrested for assaulting police at about 11pm the night before, according to lawmaker Lam Cheuk-ting.

Chung reportedly told his sons the officers said to him “this is what black cops do”, during the attack.

A police spokesman said: “The force will resolutely investigate the case fairly and squarely, with absolute impartiality.”

Tse Chun-chung, chief superintendent of the Police Public Relations Branch, confirmed that two officers captured in the footage had been arrested on Tuesday afternoon for assault occasioning actual bodily harm and placed under investigation, after which they would be suspended.

“Police will not turn a blind eye to violence, especially for officers deliberately breaking the law,” Tse said.

Chung’s sons filed a complaint to police days after the incident in late June. Tse confirmed police did not reach out to the hospital between the report and the video’s release, but denied the force had failed to do its job.

“Based on our understanding, there are usually no surveillance cameras in wards, so we will approach the hospital later on this,” Tse said.

A spokesman from the hospital said the man had been put into a “disturbed patient room” as he was emotionally unstable and agitated.

The spokesman said the patient was then guarded by the officers.

“Medical staff went to the room regularly to monitor the patient’s condition and did not find anything out of the ordinary,” he said, adding the patient had not made a complaint while at the hospital.

According to staff at the hospital, security camera footage is shown on monitors at a nursing station near where the attack took place.

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“Anyone who took a look at the screen in the nursing station could have seen it,” a source said.

Staff also said the 62-year-old had been put inside a special room with padded walls and floors normally reserved for psychotic patients, or those at risk of suicide.

In the video, which was given to Lam by the man’s sons, officers are seen applying pressure to Chung’s head and an eye as he lies helpless on a hospital trolley. They then punch him in his genitals, abdomen and face, twist his wrists and later remove his trousers.

At one point, one of the officers appears to stuff a baton, which had just touched Chung’s genitals, into the victim’s mouth.

Two other officers were seen briefly entering the ward, including the officer later arrested for not reporting the attack. The force said it would investigate the roles of other officers present at the time.

Chung’s apparent anguish appears to cause him to bang his head on a metal part of the bed, and his sons said their father had been ready to commit suicide.

Lam said the footage had been reduced from 28 minutes to the eight he made public, and had been obtained after the victim’s eldest son filed a request under personal privacy provisions with the authority.

Chung, who was reportedly drunk at the time of his arrest, was taken into custody around 11pm on June 25 on suspicion of assaulting police. The arrest was not related to any of the recent anti-government protests.

“He shouted ‘black cops’ before he was assaulted, then the police shouted, ‘this is what black cops do!’” said the younger son, relaying what his father had told him about his ordeal.

The sons criticised the officers’ “lawless” behaviour, and said they should be sent to jail.

“His hands were already tied, and he’s not even close to murder. Why should police torture an elderly citizen?” they said.

They said Chung only felt safe enough to tell them about the incident after he got home.

Chung reportedly had a broken ring finger and complained of pain in his genitals.

Lam condemned the officers for bringing shame to the already embattled force.

“This is beyond a simple assault,” Lam said. “He was so drunk. The victim might have strongly criticised police officers but there was no reason for those officers to abuse their power to torture the victim, to humiliate him so seriously.

“As police officers, they should act professionally.”

Lam also wondered if the incident was the “tip of the iceberg”, and whether anti-government protesters might have faced similar treatment.

Additional reporting by Sum Lok-kei and Clifford Lo