By Amy Qin and Tiffany May

HONG KONG — The Hong Kong police have announced their first arrest in connection with Monday’s assault on the territory’s legislative building, as protesters gird themselves for the possibility of more detentions and what they fear will be a citywide dragnet.

The police said in a statement Wednesday night that they had detained a 31-year-old man on charges that included forcible entry into the building that houses the Legislative Council, as well as causing damage to the premises and attacking the police. The man was identified only by his surname, Poon.

Several hundred demonstrators broke into the legislature on Monday night after a day of protests, centered on an unpopular bill that would allow extradition to mainland China. Some of them defaced portraits, destroyed surveillance cameras and spray-painted political slogans on walls, as riot police officers looked on. Hundreds of thousands of other people marched peacefully on Monday in a separate demonstration.

Hong Kong, a semiautonomous Chinese territory, has been roiled by protests since the city’s leaders tried last month to push the extradition bill through the legislature. The bill would make it possible for criminal suspects to be sent to the mainland, where the courts are controlled by the Communist Party, and many residents believe it would put dissidents and others in jeopardy and mark an alarming new development in the erosion of the former British colony’s civil liberties.

After enormous protests in June, which included some violent clashes with the police, Carrie Lam, Hong Kong’s chief executive, said she would suspend the bill. But demonstrators have demanded that it be fully withdrawn and that Mrs. Lam step down.

Earlier Wednesday, the police had announced 12 arrests in connection with the protests Monday, but those were connected with attempts to disrupt a flag-raising ceremony, not the attack on the legislative building.

Reporters were invited by the Hong Kong government on Wednesday to tour the ransacked building, where they saw detectives collecting boxes and bags of evidence. Billy Li, the head of the Progressive Lawyers Group, a pro-democracy association, said the police would crosscheck the evidence with their DNA and fingerprint databases. That process usually takes weeks but can be accelerated, he said.

Many of the protesters are expecting more arrests in the coming days. On messaging apps, which the largely leaderless movement has been using to coordinate its activities, tips have circulated about what to tell the police if arrested: “I have nothing to say. I demand to see a lawyer. I have the right not to unlock my phone. I need to eat, drink water, go to the bathroom, have a blanket.”

Since the clashes on Monday, questions have been raised about why the police stood by for hours as demonstrators laid siege to the walls of the building, then retreated when it became clear that they had breached the legislature’s inner chamber. Hours later, after most of the protesters had left, the police used tear gas to disperse the remaining crowd outside.

In a statement on Wednesday, the police said that a physical confrontation within the Legislative Council’s confined space could have had “unpredictable” consequences. The police have come under intense criticism for using tear gas and rubber bulletsagainst protesters on June 12.

While leaders in Beijing and Hong Kong have condemned the mostly young protesters who stormed the legislature on Monday as “extreme radicals,” many demonstrators who had not joined them said they condemned the vandalism but understood those protesters’ frustrations.

Jasper Tsang, a former president of the Legislative Council, joined the ranks of a small group of pro-Beijing lawmakers who have unexpectedly urged the government to give more consideration to the protesters’ demands. In a televised interview with a local broadcaster, Now News, on Wednesday night, Mr. Tsang raised the possibility that Mrs. Lam could grant amnesty to the protesters.

“The chief executive could consider that the whole event was caused by conflict on a deeper level,” Mr. Tsang said. “If the punishments of the young people could be pardoned, then that would be beneficial for the entire society.” Mr. Tsang also suggested that Hong Kong’s political system needed reform.

Some protesters have pointed to one of the slogans spray-painted in the Legislative Council as summing up their frustrations: “It was you who taught me peaceful protests don’t work.” They noted that nonviolent marches, some of them among the largest in the city’s history, have not pressured Mrs. Lam to fully withdraw the extradition bill or to step down, as they have also demanded.

On Thursday, the Hong Kong University of Science and Technology student union said Mrs. Lam had reached out to propose a closed-door meeting with its members, an apparent bid to follow through on her promises to improve communications with the public. But the student union said it had turned her down, saying that any such meeting must be open to the public.

“The union cannot represent all protesters, and those discussing with Carrie Lam’s government must include all sectors of society,” the student union said in a statement.

The Progressive Lawyers Group appealed to protesters to exercise self-restraint, saying in a statement that their cause “may be undermined by any unwarranted use of force.” But the statement added that “the abuse of power by those in public office does far more damage to the rule of law than disobedience of the law.”

Article originally appeared in New York Times on 4 July 2019