HONG KONG — When lawyer Hermes Chan visited a 19-year-old client in the hospital last month, he found her lying limp in bed under police watch. Bruises covered her arms, knees and shins like patchwork.

The young woman had been fleeing from riot police, Chan said, after an unauthorized protest Aug. 11. As she and dozens of others rushed into a subway station to head home, police fired pepper-spray balls from close range. An officer grabbed her by the collar, yanking her upward so that her back was exposed, before pushing her to the floor and pummeling her.

“The beating didn’t stop even when she was arrested,” Chan said in an interview, adding that his client was initially charged with unlawful assembly. A local news video capturing the melee shows groups of officers grabbing protesters and thumping them with batons.

The clash was one of many over Hong Kong’s summer of unrest to have fueled concerns about police use of force. The city’s leader has refused to allow an independent investigation into police actions, a chief reason demonstrators have taken to the streets over successive weeks to call for greater accountability, among other demands.

Chan said that when he tried to reach his client the day after her arrest, officers refused to put him through — even when he tried calling the main line of the hospital where she was receiving treatment under police watch. The young woman, whom he declined to identify by name because he said she was traumatized, was ultimately released within 36 hours.

A report from Amnesty International, released Friday local time, suggests that the woman’s experience is part of an “alarming pattern” of “reckless and indiscriminate” tactics employed by Hong Kong’s police, who have stepped up their use of force since June as unrest in the semiautonomous Chinese territory has snowballed.

Amnesty’s report, based on interviews with nearly two dozen arrested people, offers evidence of torture and other mistreatment of protesters in police detention. One man detained at a police station after a protest told Amnesty that officers took him to a separate room when he refused to answer a question, beat him severely and threatened to break his hands.

“I felt the pain in my bones and couldn’t breathe. I tried to shout but I couldn’t breathe and couldn’t talk,” the man said, according to the rights group’s report.

“Given the pervasiveness of the abuses we found, it is clear that the Hong Kong police force is no longer in a position to investigate itself and remedy the widespread unlawful suppression of protesters,” said Nicholas Bequelin, Amnesty’s East Asia director, stressing the need for an independent inquiry.

A spokesman for the Hong Kong police, in emailed comments to The Washington Post, said the police respect “the privacy, dignity and rights of the person under police custody and have a set of standard procedures in handling the person in police custody.”

Detainees in need of medical attention are brought to the nearest clinic or hospital, the spokesman added.

In response to questions on Chan’s client, the spokesman said that police “do not comment on individual cases” and that anyone who feels aggrieved about their treatment can “lodge a complaint through the existing well established mechanism.”

Hong Kong’s government and police force are now in uncharted waters — facing not only the ire of their people, but also that of human rights groups known for exposing violations worldwide, as well as the United Nations. U.N. experts last week condemned the excessive use of force by law enforcement officials.

When Hong Kong Chief Executive Carrie Lam moved to withdraw the extradition bill that sparked the protests, even her pro-Beijing political allies said she was out of step with a populace now angered more by police actions than by the shelved extradition proposal. They urged Lam to reconsider her stance that an existing police complaints mechanism, which critics charge is stuffed with pro-government loyalists and has no ability to call witnesses, is sufficient.

Hong Kong police have arrested almost 1,500 people over the course of the demonstrations. Yet the arrests and bans on large assemblies have not stopped people from taking to the streets. Protesters are planning civil disobedience actions on Oct. 1, the 70th anniversary of the People’s Republic of China’s founding. Hong Kong’s government this week canceled a harborfront fireworks show planned for that evening, citing “public safety.”

Throughout the crisis, police have defended their use of force as appropriate and effective. They point out that protesters have been illegally occupying roads, vandalizing subway stations, assaulting officers and setting fires.

“We must bear in mind, with the intense and excessive violence used by rioters, everyone in society is a loser,” Tse Chun-chung, chief superintendent of the police’s public relations branch, said on Monday. “Weeks after weeks and month after month, these radical protesters are tearing our society apart.”

Many residents, human rights groups, lawyers and others say that police have not been consistent in exercising their duties. They accuse the force of responding slowly and with restraint when pro-Beijing gangs have attacked protesters, while deploying overwhelming force against anti-government demonstrators.

Sen. Marco Rubio (R-Fla.), a longtime advocate for Hong Kong’s autonomy, told a congressional hearing this week that pro-Beijing thugs associated with organized crime and the Communist Party’s overseas influence operations had violently confronted demonstrators, journalists and passersby in recent months.

“And the police just looked on, looked the other way, and in some cases, even cooperated,” he said.

Attention is shifting to the international community’s response to the events in Hong Kong. Congress is considering a bill that would seek to punish those who suppress the city’s freedoms, and some lawmakers are pushing legislation that would ban the sale of riot-control equipment to Hong Kong police. Beijing’s encroachment on the territory’s freedoms also is expected to feature in discussions at the U.N. General Assembly this month.