At military spectacle, Xi sends a message to Hong Kong.

To the report of a 70-gun salute, 15,000 soldiers goose-stepped along Chang An Avenue — the Street of Eternal Peace — as an enormous military parade kicked off in Beijing.

The parade, commemorating 70 years of Communist Party rule in China, was overseen by the top leader, Xi Jinping, and is one of the largest in modern Chinese history. It included 100,000 performers and was the capstone of a week of events meant to celebrate the country’s rapid emergence as a global power.

In his opening speech before the parade, Mr. Xi quickly hit on the theme of Hong Kong, the semiautonomous territory that has been roiled by anti-government protests for months.

“No force can shake the status of our great motherland, no force can obstruct the advance of the Chinese people and Chinese nation,” Mr. Xi said speaking from Tiananmen, or the Gate of Heavenly Peace, which overlooks the square.

Mr. Xi said that China would “maintain the lasting prosperity and stability” of Hong Kong and Macau. He made no mention of the months of strife in Hong Kong, but his words left no mistake that Hong Kong is on the mind of Chinese leaders today.

Mr. Xi also used the occasion to emphasize his vision of narrative of national unity and rejuvenation under party rule. “No power can stop the progress of the Chinese people and the Chinese nation,” he said.

In the tradition of past parades, Mr. Xi, wearing a Mao-style suit, stood in the open sunroof of a Chinese-made Red Flag limousine as he reviewed the troops. He called out “Greetings, Comrades,” and “Comrades, you are working hard!” The troops responded in unison: “Greetings, Chairman” and “Serve the people!”

Tiananmen Square was packed with dignitaries, party members and foreign journalists. Access was tightly controlled. Many Chinese attendees were from government offices, top universities and state-owned enterprises.

“It is a great honor to attend,” said Zhou Wenli, a university physics instructor. “It’s a special chance.”

Ushers instructed the visitors on the proper demeanor they should adopt. “Remember that you’ll be on television so have a friendly expression,” one usher told a group of university administrators. “Laugh and have fun!”

Visitors needed little encouragement. They waved Chinese flags at the banks of cameras, especially one on a wire that flew back and forth across the boulevard on a wire.

The order and pageantry of the parade in Beijing offer a stark contrast to a spectacle of different sort expected in Hong Kong. Some residents of the semiautonomous territory plan to turn the holiday into a day of protest, demonstrating against the central government and calling for police accountability and democratic reforms.

For Xi Jinping, a national holiday that’s also about him.

Since the founding of the People’s Republic of China in 1949, Communist Party leaders have established a kind of liturgy for how to celebrate the anniversary, including the role of the military. This year’s anniversary, the 70th, is expected to follow that script.

But President Xi Jinping has also created new ways to put himself and his message of patriotic obedience to the fore, including in this year’s buildup to the military parade. Mr. Xi featured prominently on Monday in a recently established ritual: a ceremony in Tiananmen Square to mark Martyr’s Day, a holiday established in 2014 to honor those who have given their lives to the Communist Party’s cause. He also paid his respects at Mao Zedong’s mausoleum.

In his speech on Tuesday, Mr. Xi referred to Mao Zedong but did not mention his predecessors as Chinese leaders even as two previous presidents, Hu Jintao and Jiang Zemin, stood nearby listening to the address. Instead, Mr. Xi’s focus was on the theme of “national rejuvenation” that he has made his own since taking office in 2012.

“On this day 70 years ago on this spot, Comrade Mao Zedong announced to the world the founding of the People’s Republic of China, and the Chinese people henceforth had stood up,” Mr. Xi said. “This great event utterly transformed the tragic face of China for over a century of modern history when it was poor, weak and bullied.”

(In fact, Mao did not make his famous remark about the Chinese people standing up in his speech at Tiananmen on Oct. 1. He used a similar phrase in a speech no long before.)

“The Chinese nation advanced along the grand road toward achieving its great rejuvenation,” Mr. Xi said.

Hong Kong’s demonstrations could devolve into clashes.

Antigovernment protesters are planning demonstrations in Hong Kong on Tuesday that could descend into violence and clashes with the police.

Organizers expect tens of thousands of people to take to the streets despite the government’s refusal to grant a permit for a public assembly. Instead, bands of protesters will gather across different districts in a show of resistance to the Communist Party.

On Tuesday morning, police stopped two dozen antigovernment protesters dressed in black and holding a banner that read “End one-party dictatorship.” The protesters had tried to march to the Golden Bauhinia Square in the downtown Wan Chai district where the government was holding a flag-raising ceremony. The police used pepper spray to break them up and handcuffed a few protesters after some of them scuffled briefly with a small group of pro-government supporters.

The protesters also chanted slogans referring to the Chinese military’s June 4, 1989 military crackdown on pro-democracy demonstrations on Tiananmen Square, one of the most sensitive chapters in Chinese history

Near Hong Kong’s famed Victoria Harbour, police in green fatigues walked along the waterfront, keeping a wary eye out for protests. For security reasons, the authorities did away with an outdoor viewing area and instead had local dignitaries gather in a convention center to watch a live broadcast of the flag-raising ceremony. Two government helicopters carrying China’s national flag and Hong Kong’s flag flew over the harbor.

The city’s No. 2 leader, Chief Secretary Matthew Cheung, who presided over the event, said the government was sparing no effort to restore peace.

“Shocked and saddened by the violence that has turned the city that we call home into an unfamiliar place, Hong Kong people desperately yearn to get out of the existing gridlock,” Mr. Cheung said.

The government has worked with “its greatest sincerity” to resolve the impasse, he said, pointing to its efforts to set up channels for communication with the public.

Chinese hard-liners have long feared that hostile foreign forces are using Hong Kong as a base of subversion against the mainland and top officials do not want anything to overshadow their National Day celebrations. The Hong Kong police force on Monday arrested two high-profile democracy activists in connection with the storming of Hong Kong legislative office on July 1 by a small group of confrontational protesters.

Street violence has increased over the course of the protests, and the local police have made more than 1,700 arrests since June. Police officers have also deployed tear gas, pepper spray and rubber bullets against the demonstrators, in a use of force that many protesters have described as excessive.

The authorities closed or planned to close about a dozen subway stations in areas that protesters had said they would target.

The military parade is prepared in secret.

For weeks, details of the military parade in Beijing were guarded like a national secret. Residents near the parade route were ordered to close their curtains each night at 8 p.m. and rehearsals took place at midnight.

The tops of some of the colorful parade floats could be seen poking over the gates at Workers’ Stadium, but residents who dared to take a look were shooed away by security guards.

Ahead of the holiday, the authorities imposed measures around the country, particularly in Beijing, to ensure that the festivities were uninterrupted.

The area around Tiananmen Square was put on lockdown, and residents who live on nearby streets were told to remain at home for the parade’s duration. Passenger trains undergwent security checks, and all unauthorized flying objects — including racing pigeons — were banned.

The authorities also policied cyberspace. Internet access in the city was throttled and Weibo, the popular microblogging site, said it would delete content that “distorts” or “insults” Chinese history.

The Communist Party controls many things in China but one thing that it could not rein in today was the pollution.

Beijing woke on Tuesday to a pall of smog and dust ahead of the parade — despite the usual government diktats that have ensured blue skies on important holidays in the past.

Industries north of the Yellow River were shut down, including a glass tempering factory in Shijiazhuang, south of Beijing, which confirmed that it had closed for the holidays five days ago and will remain shut until Friday. Construction sites in Beijing also went idle. Trucks were barred from the city center.

To no avail. The air quality index reached 154, a level that is considered unhealthy. Outdoor activity is not recommended, which has been the case for several days now.

One culprit, beyond heavy polluting industries, is the weather, which the authorities cannot control. An extended dry spell winds from the south have blown in pollutants from the country’s manufacturing heartland into the basin where Beijing sits. (Northern winds have the opposite effect.)

Reporting was contributed by Russell Goldman, Gillian Wong, Keith Bradsher, Michael Ives, Andrew Jacobs, Li Yuan, Elsie Chen, Tiffany May and Elaine Yu in Hong Kong, and Christopher Buckley, Steven Lee Myers, Alexandra Stevenson, Edward Wong and Ian Johnson in Beijing. Claire Fu and Albee Zhang contributed research in Beijing.