HONG KONG — The Hong Kong government said on Friday that it would postpone the city’s September legislative election by one year because of the coronavirus pandemic, a decision seen by the pro-democracy opposition as a brazen attempt to thwart its electoral momentum and avoid the defeat of pro-Beijing candidates.

“It is a really tough decision to delay, but we want to ensure fairness, public safety and public health,” said Carrie Lam, Hong Kong’s chief executive.

She cited the risk of infections, with as many as three million or more people expected to vote on the same day; the inability of candidates to hold campaign events due to social distancing rules; and the difficulties faced by eligible voters who are overseas or in mainland China and cannot return to cast ballots because of travel restrictions.

The delay was a blow to opposition politicians, who had hoped to ride to victory in the fall on a wave of deep-seated dissatisfaction with the government and concerns about a sweeping new national security law imposed by Beijing on Hong Kong. And it was the latest in a quick series of aggressive moves by the pro-Beijing establishment that had the effect of sidelining the pro-democracy movement.

On Thursday, 12 pro-democracy candidates said they had been barred from running, including four sitting lawmakers and several prominent activists like Joshua Wong. Mr. Wong said he was barred in part because of his criticism of the new security law.

“Clearly it is the largest election fraud in #HK’s history,” Mr. Wong wrote on Twitter after Mrs. Lam announced the postponement.

Credit…Lam Yik Fei for The New York Times

Even before Friday, the city’s pro-democracy opposition had accused the government of using social-distancing rules to clamp down on the protest movement that began more than a year ago.

Earlier this week, amid reports that the vote might be delayed, Eddie Chu, a pro-democracy legislator running for re-election, said that China’s ruling Communist Party was ordering “a strategic retreat.” They “want to avoid a potential devastating defeat” in the election, he wrote on Twitter.

The explanation that Hong Kong must delay the vote because of the pandemic is likely to fall flat among the wider public, said Ma Ngok, an associate professor of political science at the Chinese University of Hong Kong.

“I think it will be seen as a kind of manipulation, that the government is afraid of losing the majority and that is why they postponed the election,” he said.

Mrs. Lam denied that the decision had been influenced by political concerns. “It is purely on the basis of protecting the health and safety of the Hong Kong people and to ensure that the elections are held in a fair and open manner,” she said.

While Hong Kong has been a world leader in controlling the coronavirus, in recent days it has seen its worst surge of infections yet, with more than 100 new cases reported daily for more than a week. The government has unfurled several new lockdown and social-distancing measures.

“We face a dire situation in our fight against the virus,” Mrs. Lam said.

Under Hong Kong law, an election can be delayed for up to 14 days if there is a “danger to public health or safety.” But Mrs. Lam postponed the election until Sept. 5, 2021, under emergency powers that allow the chief executive to make any regulations considered to be “desirable in the public interest.”

Those powers, which date to the British colonial era, were invoked last year when the government banned the wearing of masks in an effort to stem protests.

China’s central government said it supported Mrs. Lam’s decision to delay the election, the state-run Xinhua News Agency reported.

Mrs. Lam acknowledged that the move created a “rather thorny issue” under the Basic Law, Hong Kong’s constitution, which limits the terms of Legislative Council members to four years — meaning that the current lawmakers’ terms will soon expire.

That matter will be referred to the standing committee of the National People’s Congress in Beijing, which has the power to interpret the Basic Law, for a decision on how to deal with the gap, Xinhua reported.

Credit…Lam Yik Fei for The New York Times

The postponement will likely be met with criticism from the United States and other countries that have expressed sharp disapproval of China’s tightening grip on Hong Kong. This month, President Trump said that because of the national security law, the United States would begin to curb its special treatment of Hong Kong and deal with it more in line with the rest of China.

The elections “must proceed on time,” Secretary of State Mike Pompeo said on Thursday in a U.S. radio interview. “They must be held. The people of Hong Kong deserve to have their voice represented by the elected officials that they choose in those elections.”

“If they destroy that, if they take that down, it will be another marker that will simply prove that the Chinese Communist Party has now made Hong Kong just another Communist-run city,” he added.

Wang Wenbin, a spokesman for China’s Ministry of Foreign Affairs, said on Friday that the Hong Kong election was “a local election in China and is purely China’s internal affair.”

The national security law targets activity that it describes as secession, subversion, terrorism and collusion with foreign powers. It has stirred concerns in Hong Kong because it allows mainland security services to operate openly in the city and makes some speech, such as advocating Hong Kong’s independence, illegal.

On Wednesday, in a sign that officials would strictly enforce the law, the police arrested four activists, ages 16 to 21, who were accused of supporting separatism in social media posts.

And the next day, in barring the 12 opposition candidates, the Hong Kong government said that the grounds for disqualifying them included advocating for Hong Kong’s independence or self-determination, soliciting intervention from foreign governments, expressing an objection in principle to the national security law Beijing imposed last month, or vowing to indiscriminately vote against government proposals.

Opposition candidates say the moves suggested that pro-Beijing officials were concerned about a resounding defeat in the September election. Even establishment candidates have been quietly discussing the potential for a pan-democratic wave.

Elections for neighborhood-level offices, held last November, were seen as a warning: The opposition took control of 17 out of 18 district councils, which had normally been controlled by pro-Beijing parties.

This year, the opposition set its sights on a bigger target: to take at least half the 70 seats in the Legislative Council, the top lawmaking body in the territory.

Credit…Lam Yik Fei for The New York Times

While the protests have abated in recent weeks under the authorities’ crackdown, discontent with the government has remained strong since Beijing imposed the security law on Hong Kong, a semiautonomous city that maintains its own local government and legal system.

Two weeks ago, more than 600,000 people participated in the opposition camp’s primary election, despite warnings from local officials that it might be illegal. Voters generally preferred candidates closely associated with the past year’s protests.

In barring the candidates for the September elections, election officials questioned whether candidates who had previously lobbied foreign governments would continue to do so, which could potentially violate the new security law’s prohibitions on foreign influence. Another question asked was whether candidates, if elected, would veto the government’s budget. Under Hong Kong’s system, if the legislature blocks the budget twice in a row, the chief executive is forced to step down.

Kwok Ka-ki, a legislator who was one of the 12 candidates disqualified Thursday, replied that such a question was political in nature, and that he was unsure why an election official had any business asking it. “After all, this is why there are elections in the first place,” he wrote.

Just half the seats in the legislature represent geographic districts in Hong Kong, another barrier for the pro-democracy camp. The other half are functional constituencies largely set aside for candidates from various commercial sectors, which tend to vote for establishment candidates.

The opposition has pointed to other places that have held successful elections during the pandemic, including South Korea and Singapore.

“I don’t think many people in Hong Kong will be convinced,” Mr. Ma said, referring to the official justification for delaying the election. “They are allowed to go to work, take the subway, take the bus, stand in long queues and then not allowed to vote? It won’t be very convincing.”

Elaine Yu and Tiffany May contributed reporting from Hong Kong. Keith Bradsher contributed reporting, and Claire Fu contributed research, from Beijing.

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