HONG KONG — Riot police surrounded a university campus with a handful of protesters remaining inside as Hong Kong’s showdowns took a dramatic turn after a day marked by clashes, mass arrests and hundreds of injuries.

Hong Kong’s police force said Tuesday that it had apprehended 1,100 people over the past day alone, as months of violent clashes appeared to reach something of a climax. It said many of the detainees would be charged with rioting and possession of offensive weapons.

It was the largest number of arrests and injuries on a single day since the protests began five months ago.

Meanwhile, Hong Kong faced a fresh threat to its tenuous freedoms and autonomy, as the Chinese government admonished the city’s judiciary after a court overturned a ban on demonstrators wearing face masks.

The central government’s Hong Kong affairs office said that Monday’s judgment “blatantly challenged the authority” of China’s legislature and of Hong Kong Chief Executive Carrie Lam, creating “severe negative social and political impact.”

The pointed remarks were perhaps the starkest and most public instance of Beijing’s weighing in on a judicial decision in Hong Kong, which is guaranteed independent courts under the Basic Law, its mini-constitution.

The intervention underlined one of the central grievances of Hong Kong’s protest movement — perceived encroachment by the mainland government on the semiautonomous territory’s affairs.

In Hong Kong, concerns rose over the relative handful of protesters who have so far refused to leave the besieged campus of Hong Kong Polytechnic University, with authorities urging them to come out peacefully but refusing to rule out action to flush them out.

In a harbor-front promenade not far from the university, several hundred parents, teachers and sympathizers gathered for a peaceful, somber rally Tuesday evening, singing “Hallelujah” and waving cellphone lights in support of those remaining inside the university.

“We are thinking about the kids. We feel very strongly,” said Maggie Lau, 50, a retiree, adding that she felt upset and hurt by what has happened. “Who can help our Hong Kong? I don’t know. Hong Kong will disappear,” she said.

U.N. human rights spokesman Rupert Colville, speaking to reporters in Geneva, expressed concern about increasing violence by young people “who are clearly very angry, with deep-seated grievances.”

But Colville, who speaks for the U.N. High Commissioner for Human Rights, also urged authorities to “address the humanitarian situation” of the remaining protesters holed up at the university.

In Washington, the U.S. Senate, in a unanimous vote, passed legislation Tuesday aimed at protecting human rights in Hong Kong, Reuters news agency reported. The Hong Kong Human Rights and Democracy Act now goes to the House of Representatives, which earlier approved its own version of the measure.

After five months of street demonstrations calling for greater democracy and protesting what rights activists see as China’s steady encroachment on Hong Kong’s freedoms, the protests entered a new phase last week after a police officer shot an anti-government protester during a demonstration.

Protesters moved en masse into five Hong Kong university campuses, turning them into fortresses and stockpiling gasoline bombs, catapults, bows and arrows, and even javelins. Most of the campuses were cleared by the weekend, with hundreds of protesters finally congregating at the Polytechnic University.

But that may in retrospect have been a tactical error, transforming what had been an amorphous protest movement into a solid target against which the authorities could take action.

On Saturday, police began moving in, using tear gas and rubber bullets, and were met with rocks and molotov cocktails. Finally, on Monday, the confrontation reached a climax.

Police said 800 demonstrators were apprehended as they left the besieged campus of their own accord overnight. Three hundred of them were younger than 18.

“Also, some rioters were seen escaping by [rappelling] off a footbridge to getaway vehicles below. Our officers gave chase and were able to interrupt 37 of them including the drivers,” police spokesman Kwok Ka-chuen told a news conference, adding that the drivers were arrested for assisting offenders.

One protester, who spoke on the condition of anonymity for fear of arrest, said that about 50 demonstrators were arrested after a failed escape attempt. But others then rushed to a bridge and rappelled down on a rope to motorbikes waiting below.

“I followed them immediately, even though I didn’t know the drivers,” the protester said. “I knew it was pretty dangerous, but we had no choice but to trust them.”

She said that the motorbikes took three people at a time but that some were apprehended by police.

Earlier, Lam, the chief executive, said that about 100 protesters remained inside the university and urged them to “put down the weapons and come out peacefully.”

“If the protesters are coming out in a peaceful manner . . . then there is no situation when that sort of violence would happen,” she told a news conference.

But police would have to take “necessary action” if the situation changed, Lam said, adding that she was shocked that campuses had been turned into “weapons factories.”

Hospital authorities initially said 80 people reported to casualty departments around the city after the clashes. First-aid teams then went inside the campus to provide on-site treatment and sent 200 people to hospitals around Hong Kong for emergency treatment. They later updated the total number of injured protesters to 354.

Six police officers were also reported to be injured.

The influx of patients put so much strain on the city’s hospitals that patients with minor ailments were advised to seek help at general outpatient clinics or from private doctors.

A pro-democracy lawmaker, Ted Hui, entered the battered campus late Tuesday to try to persuade the remaining protesters to leave.

“The majority of them decided to leave after the discussion,” he told local media before escorting a group out. “It was not because they wanted to leave or surrender, but they were exhausted and likely lacked water and food supplies. As I know, there are around 10 to 20 unwilling to leave.”

Hui urged police not to storm the campus.

Another protester, also speaking on the condition of anonymity, said she escaped on Tuesday afternoon by climbing over a hill and crossing two highways before being picked up by a volunteer driver. She said morale was low after a tense faceoff with police the night before, especially as the protesters ran out of food and other supplies.

“Many of the front-liners haven’t slept, and many of us started to show signs of hypothermia,” she said.

Throughout the day and evening Tuesday, some more protesters were seen trickling out, only to be arrested by police. Some were pinned to the ground by officers in riot gear, while others were taken away on gurneys, one covered in foil used to treat patients at risk of hypothermia.

Images from the university showed twisted metal and chairs piled in a staircase, the remains of a barricade, with broken glass and other debris scattered all around. Graffiti was spray-painted on the walls.

Police had pledged that anyone under 18 would not face arrest: The names of those who left the campus were noted before they were released, but authorities did not rule out taking further action against them.

“We have not done any immediate arrests of these underage protesters,” Lam said. “But of course we have to reserve the right to undertake further investigation in the future.”

Sitting opposite the university, the mother of a Hong Kong University student said he has been missing since Sunday after joining the protests.

“I feel extremely worried, and I think the government are completely useless,” said the woman, who gave her name only as Mrs. Chan, adding that she feared her son was trapped inside the university.

“How can the police say this is a riot already?” she asked.

Another mother, who refused to give her name, appealed to Lam to show more understanding. “Carrie Lam says she is a mother, but the way they have treated my kids is not humanitarian,” she said.

Hong Kong’s new police chief, Chris Tang, said the purpose of the police operation was “only to maintain the law and public order.”

Chinese propaganda organs continued Tuesday to urge Hong Kong authorities to forcefully quell the unrest. “Tolerance cannot reform the rioters. Restraint cannot stop the crimes,” the Communist Party’s official People’s Daily newspaper said in a front-page editorial.

Despite the mass arrests, a message circulated on the popular Telegram messaging app Tuesday evening calling for another day of roadblocks and traffic disruptions across the city Wednesday, starting as early as 4:30 a.m.

Shih reported from Beijing. David Crawshaw, Simon Denyer and Ryan Ho Kilpatrick in Hong Kong and Shibani Mahtani in Chicago contributed to this report.

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