The financial hub has been rocked by three weeks of huge demonstrations sparked by an unpopular bill that would allow extraditions to the Chinese mainland, but on Monday that anger reached levels unseen for years.
Masked protesters mostly young and many wearing yellow hard hats broke into the legislature after hours of clashes with police.
They ransacked the building, daubing its walls with anti-government graffiti, in an unparalleled challenge to city authorities and Beijing.
In a rare report on the civic action, China's Xinhua News Agency said the Hong Kong government "strongly condemns and deeply regrets the extremely violent acts committed by some protesters".
Police had warned of an impending crackdown, and just after midnight officers moved in from several directions, firing tear gas and wielding batons as they charged and sending plumes of smoke drifting across the city.
Huge crowds of democracy activists earlier staged a march calling for Beijing-appointed Chief Executive Carrie Lam to step down and for a reverse of what they see as years of sliding freedoms.
But the atmosphere deteriorated as the day wore on, and a hardcore group of protesters breached parliament after hours of siege.
Once inside, they tore down portraits of the city's leaders, hoisted a British colonial-era flag in the main chamber and sprayed the city crest with black paint.