Amnesty International has accused Hong Kong police of using excessive force against pro-democracy protesters, in some cases amounting to torture, criticising a "disturbing pattern of reckless and unlawful tactics".
In a report based on interviews with nearly two dozen activists, most of whom were hospitalised after their arrests, the global rights watchdog said the city's police officers routinely went beyond the level of force allowed by local law and international standards.
"In an apparent thirst for retaliation, Hong Kong's security forces have engaged in a disturbing pattern of reckless and unlawful tactics against people during the protests," Nicholas Bequelin, East Asia Director at Amnesty International, said.
"This has included arbitrary arrests and retaliatory violence against arrested persons in custody, some of which has amounted to torture."
The rights group backed calls for an independent inquiry into police brutality, a key demand of protesters but one that has been rejected by government officials and police top brass.
Hong Kong's police force dismissed Amnesty's findings and rejected allegations it had used excessive force.
In a statement, police said their officers "exercise a high level of restraint at all times in the use of force".
In response to specific allegations contained within the report, police said they "do not comment on individual cases" and said those alleging abuse should make a complaint with the police watchdog instead.
Frequently violent demonstrations featuring hundreds of thousands of protesters have raged in Hong Kong for more than three months.
Anti-government protesters have hurled rocks, bottles and petrol bombs as well as used slingshots in their battles with police who have responded with tear gas, rubber bullets and water cannon.
Videos of police baton charging and beating protesters have frequently gone viral online.
Amnesty interviewed 21 people who had been arrested, 18 of whom were later hospitalised for injuries.
"Almost every arrested person interviewed described being beaten with batons and fists during the arrest, even when they were not resisting and often already restrained," the report's authors wrote.
Most interviewees reported the violence stopped once in custody.
But one detainee said he was assaulted for being uncooperative and another described seeing police shine a laser into the eye of a young detainee - a tactic protesters have employed against police.
Two defence lawyers also claimed their clients were beaten.
Multiple suspects described lengthy delays in receiving medical attention or access to defence lawyers.
Hong Kong's police denied those allegations, saying officers "respect the privacy, dignity and rights" of those arrested.
Hong Kong's protests were sparked by a now-abandoned plan to allow extraditions to the authoritarian Chinese mainland.
But after Beijing and local leaders took a hard line, the protests snowballed into a wider movement calling for police accountability and universal suffrage.
Amnesty's Bequelin said he believed the city's police "is no longer in a position to investigate itself and remedy the widespread unlawful suppression of protesters" and called for an independent inquiry.