HONG KONG — They descended before dawn, 1,000 police officers fanning out across Hong Kong to the homes and offices of opposition lawmakers, activists and lawyers. They whisked many off in police cars, often without telling relatives or friends where they were being taken.

Within a few hours on Wednesday, the Hong Kong police had arrested 53 people, searched 76 places and frozen $200,000 of assets in connection with an informal primary for the pro-democracy camp — all under the auspices of Beijing’s new national security law. In one swoop, the authorities rounded up not only some of the most aggressive critics of the Hong Kong government but also little-known figures who had campaigned on far less political issues, in one of the most forceful shows of power in the Chinese Communist Party’s continuing crackdown on the city.

The message was clear: Beijing is in charge.

The mass arrests signaled that the central Chinese government, which once wielded its power over Hong Kong with a degree of discretion, is increasingly determined to openly impose its will on the city. In the months since the law took effect, Beijing and the Beijing-backed Hong Kong leadership have moved quickly to stamp out even the smallest hint of opposition in the Chinese territory, where the streets once surged with huge pro-democracy protests.

The security law, which was enacted in June, has been the most visible tool of the crackdown. With the seeming blessing of Beijing, the Hong Kong authorities have been given the power to interpret the law as they see fit, taking advantage of vague parameters that criminalize anything the government considers to be acts of terrorism, secession, subversion or collusion with foreign powers.

The informal primary last July, for example, had little political import, since the Hong Kong government ultimately postponed the election. Even so, it provoked a coordinated show of official force on Wednesday that more than doubled the number of people ensnared under the law. And Hong Kong rounded them up while its most vocal critics, the United States and Britain, were distracted by their own political and health crises.

“The difference of the national security law from every other piece of legislation is that the national security law will not wait until the worst has happened,” said Ronny Tong, a member of the cabinet that advises Carrie Lam, Hong Kong’s chief executive. “Every single piece of national security law is aimed at preventing the occurrence of the worst.”

The Hong Kong government itself was more direct. In a statement Wednesday evening, the government said it would “take resolute enforcement action to achieve a deterrent effect.”

In a matter of months, Beijing has also upended the rules that have governed Hong Kong since the former British colony returned to Chinese control in 1997. The Chinese government bypassed Hong Kong courts in November and issued its own decision to order the removal of four opposition lawmakers. By doing so, it circumvented Hong Kong’s local constitution, which limits the Standing Committee of the National People’s Congress, China’s top legislative body, to making amendments or interpretations, legal scholars said.

The move all but obliterated the pro-democracy bloc of the city’s legislature. After the ouster, the 15 remaining opposition lawmakers resigned in protest, leaving an entirely Beijing-friendly group of lawmakers.

Beijing is reaching into nearly every sector of society. In recent months, the Hong Kong government has ordered civil servants to take oaths of office that emphasize the city is a part of China. Pro-Beijing politicians have called for reforms to the city’s independent judiciary, raising fears that it could become like the party-controlled courts in the mainland. Officials have also promised to redesign school curriculums to ensure that students are being taught “patriotism” and a sense of Chinese national identity.

For many democracy supporters, the question is not whether Beijing will assert itself again, but when.

“We cannot fantasize that, as long as we listen to the Chinese Communist Party, as long as we stop protesting in the streets, the party will let go of us,” said Li Chi-wang, a district councilor.

Many worry that Beijing will move next against the district councilors, a hyperlocal elected position, after the opposition’s landslide victory in 2019. Any mass disqualifications could leave the pro-democracy camp without a single foothold in elected office in Hong Kong.

The government has already announced plans to reform a mandatory high school civics course, known as liberal studies, that pro-Beijing figures have accused of radicalizing Hong Kong’s youth. University professors have described a chill on their campuses, as administrators try to prevent any national security violations. The legal scholar Benny Tai, who was arrested on Wednesday, was fired by the University of Hong Kong last year in relation to antigovernment protests in 2014.

Of special concern is the judiciary, considered one of the few remaining bulwarks against Beijing’s influence. In recent months, pro-Beijing newspapers have issued front-page denunciations of judges deemed overly lenient on protesters. A Chinese legal scholar called for the trial of Jimmy Lai, the pro-democracy media tycoon who was arrested in August on national security charges, to be transferred to the mainland.

The primary election, which drew more than 600,000 voters, was another red line. Hong Kong officials had said that holding the election could amount to subversion, citing opposition figures’ statements that, if elected, they would seek to use a majority in the legislature to block government proposals.

In particular, many candidates had said that they would seek to utilize a provision in Hong Kong law that forces the city’s chief executive to step down if legislators veto a proposed budget twice.

Establishment leaders suggested the opposition was foolish to challenge Beijing by seeking to paralyze the government.

“Last July both the central government and the Hong Kong government had warned these people,” said Lau Siu-kai, a former Hong Kong government official who is now a senior adviser to Beijing.

Still, many critics of the government were left reeling by the arrests, not only because of their scale, but also because — as many pointed out — the supposed offense was authorized in Hong Kong’s own law.

Legislators are “granted the right to disapprove budgets introduced by the government,” Civil Human Rights Front, a pro-democracy group, said. “Through the primary election, the candidates only exercised their rights to debate their political stance, and the electors had the freedom to elect those who are in their favor.”

But Mr. Tong, the cabinet member, said that those rights could not infringe on national security. “On the face of it,” he said, it is the right of lawmakers to veto legislation, “but if you think more about it, it is not.”

The willful vetoing of proposals without really considering them would amount to a breach of lawmakers’ duties, he added.

Officials have indicated that their work is far from finished. A senior police superintendent told reporters on Wednesday that officers might make more arrests in connection with the primary election. The Liaison Office of the Central People’s Government, Beijing’s official arm in Hong Kong, called for vigorous enforcement of the law.

“Only when Hong Kong’s national security law is fully and accurately implemented, and firmly and strictly enforced, can national security, Hong Kong’s social stability and public peace be effectively guaranteed,” the office said in a statement.

Perhaps the clearest sign of Beijing’s desire to flex its power was in whom the authorities chose to arrest.

Until Wednesday, those arrested under the national security law had largely been prominent activists, or people openly demonstrating against the government, such as a man who collided into police officers on a motorcycle while at a rally, or students who the police said had shouted pro-independence slogans.

But the latest arrests showed that the authorities were willing to punish any participation in pro-democracy activities, however mild or low profile.

Jeffrey Andrews, a social worker of Indian descent who was born and raised in Hong Kong, was known more for his work helping members of ethnic minority groups than for fiery slogans. Mr. Andrews ran in the primary and finished last in his race.

Lee Chi-yung also placed last in his region. While his opponents in the primary had emphasized their antigovernment bona fides, Mr. Lee’s campaign was devoted to a different issue: promoting accessibility in Hong Kong, in memory of his late daughter, who had used a wheelchair all her life.

“When Hong Kongers tried to express their views, whether through district council elections or primaries, the government chose not to listen,” Lo Kin-hei, the chairman of the Democratic Party, said in a news conference. “Instead, they took revenge.”

“If even a primary election can be twisted into something that can endanger national security, then this country’s national security is very fragile indeed,” he added.