Amnesty International has warned of an “alarming pattern” of violence used by Hong Kong police at anti-government demonstrations over the past three months, with officers reportedly beating protesters before and after arrest.

Based on interviews with 48 people, including lawyers and 21 arrested protesters, Amnesty International’s East Asia director, Nicholas Bequelin, said the human rights group had shown that “time and time again, officers meted out violence prior to and during arrests, even when the individual had been arrested or detained”, contrary to international human rights law.

“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 protests,” he said, including “arbitrary arrests and retaliatory violence”.

Quick Guide What are the Hong Kong protests about? Show Why are people protesting? The protests were triggered by a controversial bill that would have allowed extraditions to mainland China, where the Communist party controls the courts, but have since evolved into a broader pro-democracy movement. Public anger – fuelled by the aggressive tactics used by the police against demonstrators – has collided with years of frustration over worsening inequality and the cost of living in one of the world's most expensive, densely populated cities. The protest movement was given fresh impetus on 21 July when gangs of men attacked protesters and commuters at a mass transit station – while authorities seemingly did little to intervene. Underlying the movement is a push for full democracy in the city, whose leader is chosen by a committee dominated by a pro-Beijing establishment rather than by direct elections.

Protesters have vowed to keep their movement going until their core demands are met, such as the resignation of the city’s leader, Carrie Lam, an independent inquiry into police tactics, an amnesty for those arrested and a permanent withdrawal of the bill. Lam announced on 4 September that she was withdrawing the bill. Why were people so angry about the extradition bill? Beijing’s influence over Hong Kong has grown in recent years, as activists have been jailed and pro-democracy lawmakers disqualified from running or holding office. Independent booksellers have disappeared from the city, before reappearing in mainland China facing charges. Under the terms of the agreement by which the former British colony was returned to Chinese control in 1997, the semi-autonomous region was meant to maintain a “high degree of autonomy” through an independent judiciary, a free press and an open market economy, a framework known as “one country, two systems”. The extradition bill was seen as an attempt to undermine this and to give Beijing the ability to try pro-democracy activists under the judicial system of the mainland. How have the authorities responded? Beijing has issued increasingly shrill condemnations but has left it to the city's semi-autonomous government to deal with the situation. Meanwhile police have violently clashed directly with protesters, repeatedly firing teargas and rubber bullets. Beijing has ramped up its accusations that foreign countries are “fanning the fire” of unrest in the city. China’s top diplomat Yang Jiechi has ordered the US to “immediately stop interfering in Hong Kong affairs in any form”. Lily Kuo and Verna Yu in Hong Kong

Amnesty found that in 18 out of 21 cases, protesters in the semi-autonomous city – which was returned to China from the UK in 1997 – reported that they were admitted to hospital following arrest or detention, with injuries ranging from head wounds to fractured arms.

One protester told Amnesty that he had been beaten by special tactical officers, commonly known as “raptors”, when he was arrested in August at a demonstration in Kowloon.

“Immediately, I was beaten to the ground. Three of them got on me and pressed my face hard to the ground. A second later, they kicked my face … The same three [tactical officers] kept putting pressure on my body,” he said, adding that he then had difficulty breathing and pain in his ribcage. “They just said to me, ‘Just shut up, stop making noise.’”

In exceptional cases, a few protesters reported that they were also beaten by officers in detention and suffered “ill-treatment [amounting to] torture”. One man reported that police beat him after he refused to answer questions and threatened to “break his hands if he tried to protect himself”.

The report is the latest in a series of indictments of police violence in Hong Kong, which has helped to sustain anti-government protests for over three months.

It follows similar testimony about excessive use of police violence by democracy activists Denise Ho and Joshua Wong before a US Congressional-Executive Commission on China hearing about the Hong Kong protests this week.

Ho told legislators that it had “become a common daily scene 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”. She added that “riot police and plainclothes officers have shown no restraint while performing their duties”.

Although the chief executive, Carrie Lam, this month formally withdrew the extradition bill that first sparked demonstrations on 9 June, protesters say they will not go home until she meets all five protest demands, including an independent inquiry into police violence.

Lam has offered to initiate a “platform for dialogue” with community stakeholders as well as a review by the Independent Police Complaints Council, but protesters have said they do not trust the council, or her. This month the chief executive has experienced some of her lowest ratings since protests began. The secretary for security, John Lee, who oversees the police, has also seen his approval rating bottom out.

Mistrust in police remains widespread even as frontline protesters have escalated their tactics to include petrol bombs, bricks and in some cases even fighting with officers with sticks and hiking polls.

Protest chants have called for police to “return the eye” of one woman who was severely injured by a police bullet, while another has asked police to reveal the “missing” protesters who many believe disappeared after an intense clash at a subway station on 31 August.

Ted Hui, a Democratic legislator who has attended protest frontlines as an observer and was arrested this month for “obstruction”, told the Guardian that police use of batons – in lieu of rubber bullets – had ramped up since August, and that he had personally seen protesters hit on the head and across their body.

He said many of those arrested at anti-government demonstrations, who now number about 1,400, were not frontline protesters throwing petrol bombs but rather those standing farther back or even bystanders observing the conflict.

“I would say that many times the arrests are unlawful and arbitrary. I see the radical protesters, who really use some level of force, protest and [then] they are gone very quickly and so the police are not able to arrest them,” he said.

“Instead, the police are there to arrest other people – even those people who are peaceful protesters and they just joined the rally and they just have a mask with them and that’s it, without any other weapons. They will arrest them just because they look young or they are in black shirts or masks.”