HONG KONG — Tens of thousands of protesters thronged the streets of Hong Kong on Sunday afternoon, hours after President Hu Jintao of China swore in a new chief executive and Cabinet for the territory.

Surging down broad avenues between high-rises in a central shopping district, the protesters marched toward two government office complexes carrying a variety of banners. A wide range of causes were represented, including greater democracy in Hong Kong and calls for better state pensions and day care.

But the most common theme was derision toward Hong Kong’s new chief executive, Leung Chun-ying. Democracy activists have contended that he is ‘‘a wolf in sheep’s clothing,’’ whose sympathies for the Chinese Communist Party may lead him to roll back some of the city’s cherished civil liberties — although Leung has denied that.

People streamed out of Victoria Park, where the protest began, and into the march for more than four hours, making it one of the largest political protests in Hong Kong in the past decade — or anywhere in China, for that matter, since protests are banned on the mainland.

The Hong Kong police said that the number of people in the park at the beginning of the demonstration had been 55,000. Organizers said 400,000 people had participated in the march, including many who joined along the nearly 2-mile route.

“We worry that as he becomes our leader, he will betray our freedoms and civil rights,’’ said Juno Wu, a 24-year-old librarian.

The Hong Kong government issued a statement Sunday evening saying that it would protect civil liberties.

‘‘The government will uphold the core values of Hong Kong and protect the freedom and rights of the people,’’ the statement said. ‘‘The chief executive and his team will honor their pledge to hold themselves accountable to the people.’’

Ivan Choy, a Hong Kong politics analyst at Chinese University of Hong Kong, said that the crowd’s size relative to Hong Kong’s population of 7 million would make it harder for Leung to preserve his political legitimacy. ‘‘We have 5 percent of the population asking him to step down and focusing on his integrity,’’ Choy said.

The protest took place after Hu had already flown out of Hong Kong at midday, after the inauguration of Leung.

An unexpected element of the demonstration that may discomfit Beijing officials lies in the participation in the march of hundreds of mainland Chinese who carried banners denouncing the confiscation of their farms for government-backed real estate projects in communities near Hong Kong.

“It is not possible to protest in China, so we come here instead,’’ said a middle-aged mainlander who insisted on anonymity to avoid government retaliation.

Mainland Chinese have frequently attended candlelight vigils and other protests here, sometimes as participants and sometimes as curious onlookers, but usually at night. It is unusual for them to carry large banners through the streets in broad daylight.

Britain returned Hong Kong to Chinese sovereignty in 1997 after British and Chinese leaders promised considerable autonomy to the city until at least 2047.