Police brutality has been a core grievance for Hong Kong’s protesters, whose fury has been sustained by a regular barrage of social media clips showing officers carrying out violent arrests. Now a new investigation by Amnesty International claims the situation could be even uglier behind closed doors, accusing police of committing assaults in custody that amount to torture.

The report, based on interviews with 21 people arrested during the three months of pro-democracy protests, as well as corroborating interviews with lawyers and medical workers, found that Hong Kong police have been committing reckless and indiscriminate violence against pro-democracy protesters. Eighteen of the 21 were admitted to hospital, with injuries ranging from head wounds to fractured arms.

The alleged violence, before and after arrest, has been meted out in apparent retaliation against protesters, and has escalated in severity as the demonstrations have continued, according to Nicholas Bequelin, the group’s east Asia director.

“The evidence leaves little room for doubt — 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,” he said in a statement.

“This has included arbitrary arrests and retaliatory violence against arrested persons in custody, some of which has amounted to torture.”

Most of those interviewed were beaten before or during their arrests, but for some, the violence continued in custody, with several of protesters enduring torture or other cruel, inhuman and degrading treatment, according to Amnesty.

One man who was arrested at a protest in August told Amnesty said that, after he refused to answer a question, officers took him into a separate room and beat him severely, threatening to break his hands if he tried to shield himself from their blows.

“One flipped me over and put his knees on my chest. I felt the pain in my bones and couldn’t breathe,” the man said. An officer then pried open his eyes and shone a laser pointer into them. He was hospitalized for several days with a bone fracture and internal bleeding.

The use of laser pens by demonstrators to antagonize and disorient police during standoffs has been a source of contention for the authorities during the protests. Last month, they outraged protesters for arresting a student leader for buying 10 laser pointers, commonly used by teachers and stargazers, which officers claimed were “offensive weapons.”

Another protester said police threatened to electrocute his genitals while he was in jail, and witnessed police force another young protester to shine a laser pen into his own eye for about 20 seconds.

Much of the worst violence has been linked to special tactical officers, commonly known as “raptors.” One man, arrested at a demonstration in August, said three raptors beat him, shoved his face into the ground then kicked him in the face.

Responding to Amnesty’s allegations, Hong Kong police said used the minimum force necessary to make arrests, and had demonstrated “a high level of restraint” in response to the unrest, noting that nearly 240 officers had been injured so far.

Over the course of three months of protests, which began out of opposition towards a now scrapped bill that would have allowed extraditions to Mainland China, protesters have thrown petrol bombs at government buildings, stormed the halls of government, and lit fires on the street outside police stations. Police have responded with tear gas, batons, rubber bullets, water cannons, and bean bag rounds, arresting more than 1,300 demonstrators so far.

The violent picture painted by the report follows testimony about police brutality given to a U.S. Congressional hearing on the protests this week. Hong Kong democracy activist Denise Ho told lawmakers it is now a regular occurrence “to see youngsters being pinned to the ground, with bleeding head concussions and some even knocked unconscious, but still refused medical care by the police.”

Further ugly scenes are likely on Saturday, after pro-Beijing lawmaker Junius Ho issued a call for supporters to tear down the city’s “Lennon Walls” — the mass displays of Post-Its with pro-independence messages written on them with that have spontaneously around Hong Kong.

The controversial lawmaker applauded a mob of suspected triads who in July attacked demonstrators, in one of the most violent known episodes so far. Now, he says he hopes thousands will turn out to remove the protest displays put up by “thugs” and “cockroaches” — as he labels the protesters — in order to “cleanse the hearts of the people.”

Lennon Walls have been flashpoints for confrontation between protesters and their pro-Beijing opponents, with protesters being stabbed and punched near the displays in Hong Kong, and Chinese nationalist students repeatedly vandalizing a Lennon Wall at an Australian university campus.