HONG KONG — Hong Kong’s chief executive and leader Carrie Lam on Tuesday condemned the “extreme use of violence and vandalism by protesters” who stormed the city's Legislative Council building a day earlier.

“This is something that we should seriously condemn because nothing is more important than the rule of law in Hong Kong,” she said in a 4 a.m. news conference (4 p.m. Monday ET).

Protesters on Monday smashed their way into the building, entering the main chamber, spraying graffiti on the walls and hoisting the flag that the city used before it was governed by China.

Activists have been calling for the total withdrawal of a proposed law that would allow suspected criminals to be extradited from Hong Kong to the Chinese mainland.

Monday's demonstrators, some of whom were wearing hardhats or carrying umbrellas, spray painted slogans on the chamber's high wooden walls calling for Lam’s resignation.

The building was so badly damaged that the remaining two council meetings before the summer recess in mid-July can’t be held as scheduled, the president of the Legislative Council said.

President Andrew Leung said meeting rooms, the lobby and offices were all damaged, as well as the security system, adding that the repair is expected to take “a long time.”

Hong Kong's Chief Executive Carrie Lam at a news conference on Tuesday in Hong Kong. Anthony Kwan / Getty Images

A breakaway group of protesters broke into the building as some 550,000 demonstrators flooded the city's streets for the annual march marking the day in 1997 when the former British colony was handed back to China. Shortly after midnight local time, police used tear gas to clear the remaining protesters from the area.

This year's procession carried extra significance, coming after around a month of mass protests against a proposed extradition law. Lam said the bill will expire when the current legislative council session ends next year.

Critics of the law have said the changes would hurt the rule of law in Hong Kong, where residents can freely surf the internet and participate in public protests, unlike in mainland China. They say the law is part of attempts by Beijing to erode those freedoms, edging Hong Kong toward China's political and legal system.

On Tuesday, China’s Ministry of Foreign Affairs accused the protesters inside the Legislative Council building of “blatantly challenging the bottom line of ‘one country, two systems,’” and said in a statement that it supported “criminal prosecutions against violent criminals.”

A similar line was taken by the Hong Kong affairs office, which also said Monday’s protest at the Legislative Council building challenged the principle that allowed Hong Kong to operate semi-autonomously.

“The severe illegal act had tramped Hong Kong’s rule of law, damaging the city’s social order and fundamental interests,” the Xinhua news agency quoted a spokesman as saying.

The former British colony returned to China 22 years ago under a "one country, two systems" formula that allows freedoms not enjoyed in mainland China, including the freedom to protest and an independent judiciary.

Hong Kong police defended their withdrawal from the building during the protest.

Protesters spray painted walls and draped a colonial Hong Kong flag in the Legislative Council chamber on Monday in Hong Kong. Anthony Kwan / Getty Images

“My officers had no choice but to temporarily retreat, to do a regrouping and to do some redeployment to take back LegCo later on,” said Police Commissioner Lo Wai-chung, referring to the Legislative Council.

Pro-democracy supporters have said they suspected that police may have deliberately held back, giving China a reason to clamp down more strongly on protesters.

“Have you seen this in any country or city where an important building like the Legislative Council building was attacked and the police was actually present, and they did nothing to stop it?” Martin Lee, a veteran pro-Democracy campaigner and founding chair of the Democratic Party of Hong Kong, said in an interview with the BBC.

Veta Chan reported from Hong Kong, Rachel Elbaum from London.