HONG KONG — Thousands of police officers flooded onto Hong Kong's streets Thursday, stopping and searching people en masse, to thwart demonstrations planned for China's National Day to protest Beijing's tightening control of the city.

The scenes underscored the government's determination to stop any kind of protest, even peaceful, and showed the near-impossibility of staging the sort of massive street demonstrations that have been a part of Hong Kong's unique civic life for years.

Officers were stationed on virtually every corner of the Causeway Bay shopping district, searching everyone from the young to the elderly. During the course of the afternoon, police cordoned off dozens of people, lined them up against a wall, handcuffed them and marched them away. By 6 p.m., police said they had arrested 69 people, including two local legislators known as district councilors.

Stopped from marching, groups of protesters, along with regular residents, would break into chants popularized during street protests last year — including “Liberate Hong Kong, Revolution of our Times” — while jeering at police.

In a statement, police described that slogan as tantamount to inciting people to “commit secession” and as a possible breach of the national security law.

Others staged more-subtle forms of protest, conspicuously reading the pro-democracy Apple Daily newspaper — whose founder Jimmy Lai was arrested under the national security law — in front of officers.

Police swooped in to surround any substantial gatherings, warning those chanting that they were in violation of the draconian new national security law. Passersby and protesters fled into the surrounding shops and malls.

The Civil Human Rights Front, a pro-democracy group that organized million-person marches last year, had asked Hong Kong police for authorization to hold a rally on National Day, which is typically marked in Hong Kong by large pro-democracy marches. Officials rejected its application, citing the coronavirus pandemic and violence at previous marches.

Calls continued online, however, appealing for Hong Kong people to come out and push back against the Chinese Communist Party’s tightening rule and the swift erosion of the city’s once-famous freedoms.

Last year, on the 70th anniversary of the founding of the People’s Republic of China, Hong Kong police for the first time shot a protester, and fired more than a thousand rounds of tear gas across the city.

Ahead of Thursday’s planned march, Hong Kong police moved to arrest at least five people who they alleged were inciting others to protest and commit arson — hoping to stave off the demonstrations. According to local media reports, more than 6,000 officers were deployed across the city, one of the largest showings of force since protests erupted in June 2019.

Protesters also hoped to raise awareness of the plight of 12 Hong Kong activists, held incommunicado in mainland China. The 12 were intercepted by the Chinese coast guard while fleeing Hong Kong for Taiwan, hoping to seek refuge on the self-governing island.

All of them had been previously arrested in Hong Kong in connection with the pro-democracy protests, including one under the national security law. On Wednesday, they were formally arrested and prosecuted under Chinese law, moving them deeper into the opaque and politically influenced legal system on the mainland.

Beijing in late June made the stunning move of passing a national security law by fiat in the hopes of suffocating the street protests that had rocked Hong Kong for eight months until the pandemic hit.

The new law overrode all of the semiautonomous city’s local procedures and greatly raised the stakes for protesting, with the broadly worded crimes of “secession,” “subversion of state power,” “terrorism” and “foreign interference” to be punished by life in prison.

Hong Kong already had its own laws, including against unlawful assembly and rioting, that had been used to arrest some 10,000 protesters since June 2019.

In a speech marking National Day, Hong Kong Chief Executive Carrie Lam said it was “obvious to see that stability has been restored to society, while national security has been safeguarded.”

Lam added that Hong Kong must closely adhere to the “one country” principle — its status as a territory under Beijing’s rule — to keep the city “moving forward.” Under the “one country, two systems” framework that was meant to preserve Hong Kong’s semiautonomous status until 2047, the city is meant to have an independent judiciary, freedom of speech, freedom of the press and other basic rights that are absent on the mainland.

On Thursday, it was plain that many did not share Lam’s view, and there was widespread anger on display, much of it directed against police

Residents stopped by police officers frequently shouted at them and questioned their authority. Chants of “black cops” were periodically hurled at the lines of officers.

At one point, a battery-operated speaker left on the sidewalk started playing “Glory to Hong Kong,” the unofficial protest anthem. People began cheering and clapping before officers rushed to the scene, kicked the speaker over and turned it off.

The Hong Kong government has not only refused to open an independent inquiry into its conduct in the protests but awarded a record number of honors to police officers. Medals of honor are given out every National Day to exemplary citizens, which this time included 94 awards for the Hong Kong police force alone.

Stephen Lo, the former police chief who was added to a sanctions list by the U.S. government, was given the second-highest award for leading the force to “rise to countless challenges, uphold the rule of law” and maintaining Hong Kong’s safety.