Two Hong Kong police officers were arrested Tuesday after a video emerged of them beating an older man on a hospital trolly, heaping further pressure on a force already facing accusations of brutality.
The footage, recorded in late June, shows two uniformed officers assaulting the man with batons and holding a cloth over his mouth.
There are no other people in the room as the officers take turns to abuse the patient over several minutes.
Police said the man was under arrest at the time.
"It is clear that the actions committed by the police officers concerned are unlawful," said police spokesman John Tse, adding that police viewed the video for the first time on Tuesday.
"So far, two involved officers were arrested for assault occasioning actual bodily harm."
He vowed a full, independent probe, insisting "police officers are never allowed to use abusive force for their own sake".
The incident is likely to further fuel anger towards the stretched police force.
Pro-democracy protesters who have staged weeks of rallies throughout the city have called for an independent inquiry into the police response, which has included frequent use of tear gas and rubber bullets.
The force has become the loathed face of the Hong Kong government, and small groups of hardcore protesters have frequently clashed with frontline officers./AFP