The riot police left the legislative building as protesters charged.

A few hundred protesters stormed into Hong Kong's legislative complex on Monday, pumping their fists in the air, after riot police appeared to back away from a confrontation, leaving the compound hours after the core group of demonstrators started bashing their way in.

Protesters wearing safety helmets and masks had been steadily destroying the facade of the Legislative Council, leaving broken glass and torn metal panels in their wake, hours after the government held a ceremony commemorating the 22nd anniversary of the territory's return to China from Britain. Using metals bars and makeshift battering rams to break the building's outside glass walls and doors, some protesters entered the building and forced open metal roller shutters that sealed the entrance to the lobby leading to the legislative chambers. 

For hours, riot police with gas masks and shields had guarded the facility from within the building, holding up signs warning the protesters that they would use force if the demonstrators charged.

Then around 9 p.m., as protesters appeared ready to breach the shutters, the riot police officers dispersed and were seen marching toward the central government building. Once inside, the protesters sprayed messages on the walls calling for protesters arrested last month to be released. "Murderous regime," read another message. They built barricades inside using materials they had scavenged and brought, including umbrellas and metal gates and destroyed surveillance cameras. Then they filled the legislative chambers, defaced portraits of leaders and spray-painted slogans on the desks and walls.

"I am inside here to use more extreme ways for the government to accept our demands," said one protester who identified himself as a 17-year-old high school student, and whose name is being withheld. He decided to enter the building, but insisted he had not taken part in any destruction. "There is no one here who isn't scared of being arrested. I only have a bottle of water and a helmet, but the police have tear gas and pepper spray."

The protest was quickly turning into a broad repudiation of Chinese rule, with demonstrators tearing up copies of the Basic Law, a mini constitution that took effect in 1997 and governs Hong Kong's relations with Beijing, and calling for free and direct elections. One group raised a British-colonial-era flag.

"I want universal suffrage!" one protester led others in chanting.

Protesters walked through dark hallways littered with broken glass near the doorways. 

The police and government earlier said that they condemned the violence at the legislature and that officers were exercising restraint.

But early in the day, hundreds of riot police officers had used batons and pepper spray to beat back protesters at a different site — near a government flag-raising ceremony attended by the city's chief executive, Carrie Lam. 

At the handover of Hong Kong to China's control in 1997, the Chinese government agreed that Hong Kong could retain its justice system and protections for civil liberties for 50 years, under a philosophy commonly known as "one country, two systems." Protesters today are angry because they see Ms. Lam's pushing of a bill that would open the way to extradite suspects to mainland China as giving up those rights to Beijing. 

[Read more about China's promises to preserve Hong Kong's freedoms.]

The demonstrations are a challenge to China's leader, Xi Jinping.

Huge crowds of demonstrators have taken to Hong Kong's streets in the past several weeks, protesting a bill that would allow extraditions to mainland China. The protests forced Mrs. Lam to suspend the bill but demonstrators want a full withdrawal and for her to resign. 

The turnout of protesters on Monday was among the largest attempts to disrupt the Hong Kong government's most important annual political event. It underscored the deepening anxiety that many in Hong Kong feel about the erosion of the civil liberties that set the city apart.

Monday's protests, which also fell on the 98th anniversary of the founding of the Communist Party of China, were a direct challenge to President Xi Jinping and his increasingly authoritarian policies.

Analysts said the chaos risked giving Mr. Xi an opportunity to justify his tough approach.

"If it gets really violent, the risk is that Beijing has a good excuse to become even more uncompromising," said Jean-Pierre Cabestan, a political scientist at Hong Kong Baptist University. "Xi can put even more pressure on Carrie Lam not to make any concessions."

The attack on the legislative complex exposed divisions among the protesters.

Protesters who joined the demonstration outside the legislature said they were frustrated that the government was not listening to their concerns. "Friends, don't leave," read the signs many were waving. "People of Hong Kong, don't give up," 

This was the latest instance in which a group of predominantly younger protesters have taken measures that test the boundaries of civil disobedience in this usually orderly financial hub. In recent weeks, to protest the extradition bill and what they saw as a heavy-handed police response, the protesters have twice besieged the city's police headquarters and sought to disrupt government services. 

The protesters said they chose to descend on the Legislative Council because the police prevented them from getting close to the site of the government's flag-raising ceremony that morning. 

Several protesters said that while they did not personally plan to break into the complex, they supported those on the front lines who did. Peaceful protest methods were ineffective, they said, and they increasingly felt open to a more confrontational approach if it would help to protect Hong Kong's freedom and relative independence from Beijing.

"We have been too peaceful for the past few times, so the police think we are easily bullied," said Natalie Fung, 28, who was outside the legislative complex supporting the protesters with food and drinks. "The younger people are risking their safety and their futures for us."

Not all protesters supported the handful who attacked the Legislative Council. Several democratic lawmakers tried to stop the protesters by positioning themselves between the demonstrators and the building, even physically blocking the makeshift battering rams at times — but were eventually pushed aside.

Claudia Mo, who was among the pro-democracy legislators attempting to stop the protesters, said she thought the violence was an expression of how desperate the younger protesters had become in the face of a government that they thought had not done enough.

"I'm extremely worried because the young really seem like they have nothing to lose, when they have a lot to lose," Ms. Mo said. "It's their Hong Kong they're fighting for. It's their future and they need to take their future into account."

Samson Yuen, a political scientist at Lingnan University in Hong Kong who studies social movements and identity politics, said that many protesters who felt like they were facing repression could set aside their ideological differences about the use of violence. "People understand that they need to band together in order to avoid being fragmented."

In the morning, police used batons and pepper spray to beat and push back demonstrators who tried to march to the convention center. Protesters who had been hit with pepper spray stumbled to seek help at medical stations set up by supporters. Some poured water over their bodies. 

The government said around midday that demonstrators had attacked police lines and thrown an unidentified liquid at officers. Some officers reported difficulties breathing and irritated skin, and 13 were sent to the hospital, the government said in a statement. The police also said that some protesters had scattered lime powder at the police, injuring officers.

"Police strongly condemn such illegal acts and will stringently follow up," it said.

The city's embattled leader promised to be "more open and accommodating." 

Mrs. Lam, the city's chief executive, sought to strike a conciliatory note on Monday morning, pledging that she and her government would be more responsive to public sentiment. She was earlier criticized for insisting on pushing the unpopular legislation through despite an intense public outcry.

"I will learn the lesson and ensure that the government's future work will be closer and more responsive to the aspirations, sentiments and opinions of the community," Mrs. Lam said at the official ceremony commemorating the handover anniversary. "The first and most basic step to take is to change the government's style of governance to make it more open and accommodating." 

Local television news channels broadcast a startling split screen. On one side, Mrs. Lam and officials from Hong Kong and mainland China clinked champagne flutes in a toast to a unification, on the other riot police clashed violently with protesters. 

Mrs. Lam said she would make more time to meet with people from different political backgrounds and reach out to the city's youth. She said that Hong Kong's economy could feel the repercussions of a protracted trade war between the United States and China and urged Hong Kong residents to work with the government on managing the impact of the trade dispute and addressing the housing shortage and other issues.

The broad public anger has already forced Mrs. Lam to suspendthe proposed legislation, but demonstrators want it to be fully withdrawn and have also turned their scrutiny on the police, whom they say acted with excessive force in dispersing a June 12 protest. A march was planned for later in the day that pro-democracy organizers said was expected to draw a large turnout.

'Carrie Lam, step down,' protesters elsewhere chanted in a peaceful march.

Separately, hundreds of thousands of other protesters, including families and children, marched through nearly 90 degree heat on Monday afternoon to fill the streets of downtown Hong Kong in a separate demonstration calling on the city's leader to resign.

Protesters carried signs saying "Free HK Democracy Now," and "Hong Kong Fights For Democracy." The march began at Victoria Park, where a few people handed out yellow signs urging people to "stand firm and investigate police violence."

"Carrie Lam, step down, get some dignity for yourself," said Lo Woon-fun, 84, who was sitting under a small umbrella in the muddy field at the beginning of the march. "I came out today because I want to tell Carrie Lam that despite my old age, I still come out to demand she step down." 

"I have come here because of the future generation of ours. I want them to live a good life as I have," she said.

Organizers said late Monday that 550,000 people had joined the march, setting a record for the annual event.

Members of the labor union for Postal Service workers carried a large printed banner that read: "When a million people walk against the mainstream, it's inhumane to neglect it," referring to a massive protest in June.

Daniel Victor, Alexandra Stevenson, Mike Ives, Tiffany May, Katherine Li, Javier Hernandez, Austin Ramzy, Gillian Wong and Ezra Cheung contributed reporting.