Protesters daubed the Hong Kong parliament with graffiti and raised the former British colonial-era flag on the anniversary of the citys handover to China.
The semi-autonomous financial hub woke up to the aftermath of a night of unprecedented anti-government protests.
The parliament was ransacked and China has now called for a criminal probe into the unparalleled challenge to its authority.
Demonstrators have been agitating for weeks over a bill that would allow extraditions to the Chinese mainland.
The issue has now become a lightning rod for resentment towards Beijing, who now have sovereignty over the city.
On Monday night – the 22nd anniversary of the handover to China – demonstrators raised the former British colonial flag in the Hong Kong legislative chamber.
They also scrawled messages such as Hong Kong is not China on the walls and defaced the citys seal with spray paint.
Police charged the building and used tear gas against the activists.
The territorys leader Carrie Lam condemned the extreme use of violence and vandalism by protesters.
Beijing also wasted no time in asking Hong Kong to investigate the criminal responsibility of violent offenders.
They have also told the UK to know its place and stop meddling in Hong Kong affairs after Foreign Secretary Jeremy Hunt said Britain remained unwavering in its support for the territory.
He said his heart goes out to the activists who are fighting to keep alive freedoms they have long-enjoyed.
Mr Hunt added: When I look at those situations that weve just seen, and those terrible scenes in Hong Kong, my heart goes out to people who do have to fight for their freedoms and who are worried they could lose a very, very precious way of life.
I dont support violence in any circumstances but I understand their worries about changes that are happening in Hong Kong.
The territory has seen a number of mass protests over the last three weeks.
The rallies – including a huge pro-democracy march on Monday – have been largely peaceful while calling on the citys Beijing-appointed Lam to resign.
But they have failed to win concessions, with Lam refusing to permanently shelve the extradition law or step down.
By Monday some hard-line protesters appeared to have reached breaking point and stormed the legislature.