Hong Kong's leader on Tuesday condemned "the extreme use of violence" by masked protesters who stormed and ransacked the city's legislature in an unprecedented challenge to Beijing's authority.
The semi-autonomous financial hub has been thrown into crisis by weeks of massive demonstrations over a bill that would allow extraditions to the Chinese mainland.
But on Monday -- the 22nd anniversary of the city's handover to China -- anger spilled over as groups of mostly young, hardline protesters, broke into the legislative council.
They hung the city's colonial-era flag in the debating chamber, scrawled messages such as "Hong Kong is not China" and defaced the city's emblematic seal with spray-paint.
Police charged into the building shortly after midnight.
The events pose an unparalleled challenge to Chinese leader Xi Jinping, who has so far left Lam to handle the protests.
In a pre-dawn press conference, Lam, the city's Beijing-appointed chief executive, described the scenes of vandalism as "heartbreaking and shocking".
"This is something we should seriously condemn because nothing is more important than the rule of law in Hong Kong," she said, with the city's police chief Stephen Lo by her side.
Lo added: "Protesters' violent acts have far exceeded the bottom line of peaceful expressions of demands."
The financial hub has been rocked by massive protests over the past three weeks as anger against the pro-Beijing government has boiled over.
The rallies -- including a huge pro-democracy march on Monday -- have been largely peaceful while calling for Lam, whose approval ratings are at a record low, to resign.
But they have failed to win many concessions, with Lam resisting calls to permanently shelve the extradition law or step down.
On Monday, some hardline protesters appeared to have reached breaking point, saying they felt compelled to storm parliament because their concerns were going unheard.
"We have marched, staged sit-ins... but the government has remained unmoved," Joey, a 26-year-old protester, told AFP as she walked over shattered glass inside the building.
"We have to show the government that we won't just sit here and do nothing."
"There are no rioters, just tyranny," read one banner hoisted above the podium.
The legislature was closed on Tuesday, as police inspected the damage inside the building, while workers waited outside to begin a clean-up.
Pro-democracy lawmaker Eddie Chu was stopped from entering his office as police cordoned off the debris-strewn building with barricades.
"The police said the whole place is a crime scene. So now the whole place is basically taken over by the police," Chu told reporters outside.
Under the terms of Hong Kong's handover from British to Chinese rule in 1997, the city enjoys rights and liberties unseen on the autocratic mainland. But many residents accuse Beijing of reneging on that deal with the help of unelected leaders.
On Monday, the United States voiced its support for the protesters, with President Donald Trump saying the demonstrators were "looking for democracy".
"Unfortunately, some governments don't want democracy", he added, in an apparent swipe at Beijing.
Chinese state media dismissed the protests as "mob violence".
"Chinese society is all too aware that a zero-tolerance policy is the only remedy for such destructive behavior", the state-run Global Times daily wrote in an editorial.
"Otherwise, and without this policy, it would be similar to opening a Pandora's Box, upending social disorder," it warned.
Britain's Foreign Secretary Jeremy Hunt expressed "unwavering" support for "Hong Kong and its freedoms" but urged restraint from protesters.
The increasingly hardline tactics have divided Hong Kongers, with some calling for the protesters to focus on finding common ground with government supporters.
Brokerage executive Chris Cheung told AFP the decision to attack the legislature was "unwise and unnecessary".
"Violence can't change the people's attitude, only reason will," he said.
Outside the legislature, clean-up volunteer Blue Wong told AFP she was "angry with the government for pushing the young generation to such a stage".
"I won't criticize or judge the youngsters for what they did.... Even though their actions are violent, their hearts are gentle", she said.
"They are fighting for a better Hong Kong."