Protesters raised the old British colonial flag in the Hong Kong legislative chamber amid ugly scenes on the 22nd anniversary of the territory’s return to Chinese rule.
Police used tear gas against activists who had occupied the legislative council building and painted pro-democracy slogans on the walls.
The territory’s leader Carrie Lam condemned the “extreme use of violence and vandalism” by protesters.
The scenes follow unrest in the former colony over a controversial extradition law and a feud between Foreign Secretary Jeremy Hunt and the Chinese foreign ministry.
Officials in Beijing urged the UK to “know its place and stop meddling in Hong Kong affairs” after Mr Hunt said Britain will remain “unwavering” in its support for the territory.
Ms Lam contrasted the protests at the assembly with a “peaceful and generally orderly” march which routinely takes place on July 1 – the anniversary of the handover.
“The second scene that we have seen, which really saddens a lot of people and shocks a lot of people, is the extreme use of violence and vandalism by protesters who stormed into the Legislative Council building over a period of time.
“So this is something that we should seriously condemn, because nothing is more important than the rule of law in Hong Kong.
“I hope the community at large will agree with us that with these violent acts that we have seen, it is right for us to condemn it and hope society will return to normal as soon as possible.”
Mr Hunt told Sky News that his “heart goes out” to the protesters in Hong Kong.
“When I look at those situations that we’ve 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,” he said.
“I don’t support violence in any circumstances but I understand their worries about changes that are happening in Hong Kong.”
Demonstrations have been building for weeks in protest against moves by the Hong Kong government to change the extradition laws to allow suspects to be sent to China to face trial.
The proposal has awakened broader fears China is eroding the freedoms and rights Hong Kong is guaranteed for 50 years after the handover under the “one country, two systems” framework.
Artist and activist Ai Weiwei told BBC Newsnight the situation in Hong Kong was “not normal”.
Referring to Tiananmen Square, he told the programme: “China has a record to use force 30 years ago to crash down the most peaceful demonstration by students.
“If you see what happens in Hong Kong today, the students are very rational and peaceful.”