Hong Kong — Hundreds of protesters in Hong Kong swarmed into the legislature's main building Monday night, tearing down portraits of legislative leaders and spray-painting pro-democracy slogans on the walls of the main chamber. Frustration was mounting over a lack of response from the administration to opposition demands.

Police carrying riot shields and firing tear gas moved in shortly after midnight to clear surrounding streets but appeared to have paused outside the legislative building. A spokesman had earlier broadcast a warning that "appropriate force" would be used in the clearance operation, but there was no immediate word on any arrests or injuries.

Video and images showed police advancing toward the legislature and firing tear gas at protesters near the government headquarters. The crackdown began around midnight.

Police fire tear gas at protesters near the government headquarters in Hong Kong on July 2, 2019, on the 22nd anniversary of the city's handover from Britain to China. Anthony Wallace / AFP/Getty Images

The flashing blue and red lights of dozens of police vans and buses lit up the abandoned streets leading to the legislature.

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The sharp escalation in tactics came on the anniversary of the former British colony's return to China, a city holiday, and reflected mounting frustration with Hong Kong's leader for not responding to protesters' demands after several weeks of demonstrations.

The protesters whacked away at thick glass windows until they shattered and broke and pried open steel security gates and propped them open with barricades to get inside. Police in riot gear retreated as the protesters entered about 9 p.m., avoiding a confrontation and giving them the run of the building.

A shot inside the Legislature Building in Hong Kong during the protest on July 1, 2019. Ramy Inocencio CBS News

CBS News producer Chris Laible said the demonstrators, mostly young people, earlier erected barricades at building exits where they thought police would come out. They propped doors and gates to the building open with any metal objects they could find, and used umbrellas to try to block the view of police inside the building. Police shot pepper spray through a hole in the door of the building made by the protesters, which drove them back for a while. But hours later angry demonstrators swarmed into the legislature after prying open metal security curtains. Police appeared to back off as the protesters came in, apparently to avoid a confrontation.

WATCH: @CBSNews was inside Hong Kong’s Legislative Council. See what we saw here. We followed angry protestors who stormed the building. We were told police were coming to take it back. HK’s pan-Democrat’s have called on the city’s leader Carrie Lam to resign. #HongKongProtests pic.twitter.com/FTnJsLno1T — Ramy Inocencio 英若明 (@RamyInocencio) July 1, 2019

The demonstrators stood on lawmakers' desks in the main legislative chamber, painted over the territory's emblem high up on a wooden wall and wrote slogans calling for a democratic election of the city's leader and denouncing now-suspended extradition legislation that sparked the protests. Many wore yellow and white helmets, face masks and the black T-shirts that have become their uniform. Police announced about 10:30 p.m. that they would clear the area, asking protesters to leave.

The actions prompted organizers of a separate peaceful march against the extradition bill to change the endpoint of their protest from the legislature to a nearby park, after police asked them to either call it off or change the route. Police wanted the march to end earlier in the Wan Chai district, but organizers said that would leave out many people who planned to join the march along the way.

Police estimated 190,000 people joined the peaceful march, the third major one in as many weeks. Organizers estimated the number at 550,000.

Anti-extradition protesters use makeshift shields to defend themselves during a clash with police outside the Legislative Council Complex ahead of the annual flag raising ceremony of 22nd anniversary of the city's handover from Britain to China on July 1, 2019 in Hong Kong, China. Getty

Hong Kong has been wracked by weeks of protests over a government attempt to change extradition laws to allow suspects to be sent to China to face trial. The proposed legislation, on which debate has been suspended indefinitely, increased fears of eroding freedoms in the territory, which Britain returned to China on July 1, 1997.

CBS News correspondent Ramy Inocencio reported from the melee that both the combative protesters and the much larger group marching through Hong Kong's streets — said by organizers to be about 550,000-strong — were venting anger at the city's leader, Carrie Lam, and by extension her superiors in Beijing. Lam backed controversial changes to Hong Kong's extradition law that would let China transfer anyone accused of a crime in Hong Kong into the mainland's opaque court system.

A shot inside the Legislature Building in Hong Kong during the protest on July 1, 2019. Ramy Inocencio CBS News

Mounting frustration

The annual march was larger this year because of the simmering anger over the proposed extradition bill. Two marches in June against the legislation drew more than a million people, according to organizer estimates.

The government has suspended debate on the bill indefinitely, but protest leaders want it formally withdrawn and for Lam to resign. They also are demanding an independent inquiry into police actions during a June 12 protest, when officers used tear gas and rubber bullets to disperse protesters who blocked the legislature on the day debate on the bill had been scheduled to resume.

The police say the use of force was justified, but have since adopted softer tactics, even as protesters besieged police headquarters in recent days, pelting it with eggs and spray-painting slogans on its outer walls.

Earlier, protesters demanding Hong Kong's embattled leader step down clashed with police outside a flag-raising ceremony marking the anniversary of the former British colony's return to China. Lam pledged to be more responsive to public sentiment.suspended debate on the bill

Police used riot shields and pepper spray to push back hundreds of helmeted protesters who tried to advance down closed streets toward the harborfront ceremony venue, where the Chinese and Hong Kong flags were raised together and two helicopters and a small flotilla passed by.

At the ceremony, Lam said a series of protests and marches that have attracted hundreds of thousands of students and other participants in recent weeks had taught her that she needs to listen better to the youth, and Hong Kong's people in general. Lam has come under withering criticism for trying to push through the legislation.

"This has made me fully realize that I, as a politician, have to remind myself all the time of the need to grasp public sentiments accurately," she said in a five-minute speech to the gathering in the city's cavernous convention center.

She insisted her government has good intentions but said she "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."

Security guards pushed pro-democracy lawmaker Helena Wong out of the room as she walked backward shouting at Lam to resign and withdraw the "evil" legislation. She later told reporters she was voicing the grievances and opinions of the protesters, who could not get into the event.

New this morning in Hong Kong. Police cordoned off this entrance protestors used to swarm into LegCo last night. The city takes stock of the damage today - physically, psychologically, economically. CE Carrie Lam says she hopes the city will return to normal. #HongKongProtests pic.twitter.com/LqllucWxBC — Ramy Inocencio 英若明 (@RamyInocencio) July 1, 2019

The following morning, Lam said she was hoping Hong Kong would return to normal.