Video report by ITV News Asia Correspondent Debi Edward

Hong Kong's leader Carrie Lam has condemned the "extreme use of violence" by protesters and has been backed by China's foreign ministry.

Hundreds of pro-democracy demonstrators caused carnage after smashing their way into the Hong Kong government's legislative council and then breaking into the chamber on Monday night.

Their acts of vandalism, sparked by a divisive extradition bill, saw portraits of lawmakers defaced, pro-democracy slogans spray-painted onto walls and the old British colonial flag raised before they were dispersed by police using tear gas.

At a 4am press conference today, Ms Lam described the storming of the legislature as “heartbreaking and shocking”.

China's foreign ministry also criticised the occupation and vandalization as "serious illegal acts that trample on the rule of law and endanger social order."

A statement carried by the state-run Xinhua news agency, the Chinese government’s liaison office in Hong Kong further added: ''Their violent acts are an extreme challenge to Hong Kong’s rule of law. It is totally intolerable.”

Plain-clothed police officers investigate near the broken glasses outside Legislative Council building in Hong Kong. Credit: AP

However Downing Street this morning sent a strongly worded message to China not to use the protests in Hong Kong as a "pretext for oppression".

Prime Minister Theresa May's official spokesman told a Westminster briefing: "We are very concerned about the violence we have seen on all sides during the protests.

"We urge the authorities not to use last night's events as a pretext for oppression, but to address the deep-seated concern by the people of Hong Kong that their basic freedoms are under attack."

The comments come shortly after The Foreign Office rejected Chinese accusations that Britain is "interfering" with the internal affairs of Hong Kong.

Foreign Secretary Jeremy Hunt said Britain will remain "unwavering" in its support for Hong Kong.

He said: "Whilst we condemn the violence we saw - that is never acceptable - we understand the deep-seated concerns that people have about their freedoms and why they would want to march, and addressing those concerns is the way to deal with this issue."

A woman cleans up a makeshift memorial near government headquarters in Hong Kong following the violent clashes with police. Credit: AP

Tens of thousands initially took to Hong Kong's streets on Monday, the 22nd anniversary of the transfer of the former British colony to China, in peaceful protests.

Unhappiness over the controversial extradition bill centres around fears it could be used by Beijing to seize anyone it likes who sets foot in Hong Kong — from a normal resident to a political dissident - and cart them off to mainland China to be dealt with.

There is also a broader concern China is eroding the freedoms and rights Hong Kong is guaranteed for 50 years after the handover under a “one country, two systems” framework.

A splinter group of hundreds and possibly thousands of protesters chose to attack the Legislative Council building.

The carnage left behind following yesterday's violent protests. Credit: AP

Following a three-hour stand-off, the demonstrators vacated the building as riot police cleared surrounding streets with tear gas and then moved inside.

Once outside they were met by lines of riot police who let off tear gas in the direction of the protesters as they tried to clear them from the area.

In response items such as chairs could be seen being thrown towards the police response teams.

Many wore yellow and white helmets, face masks and the black T-shirts that have become their uniform.

Watch the moment protesters vandalise the legislative chamber

Protesters say they want a full withdrawal of contentious extradition legislation and the resignation of city leader Ms Lam.

Speaking at a pre-dawn press conference she remained defiant and said she hoped the wider Hong Kong community would agree their actions should not be approved.

Ms Lam noted that two different protests had taken place on Monday, and disputed complaints officials have not responded to demonstrators.

She said the government has explained that by suspending the bill with no timetable or plan to revisit it, the legislation would die at the end of the current legislative session in July 2020.

Police officers in anti-riot gear cleared protesters. Credit: AP

She contrasted the protests at the assembly with a "peaceful and generally orderly" pro-democracy 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."

A spokesman for China's foreign ministry, Geng Shuang said China's central government strongly supports Hong Kong's government and its police force in dealing with the incident in accordance with law.

protesters inside the legislative chamber. Credit: AP

Away from the chaos inside the Legislative Council, protesters also took over key roads before a huge pro-democracy rally - aerial video footage showed tens of thousands snaking their way through the city ahead of the march.

Hong Kong officials estimated around 190,000 people joined the peaceful march, the third major one in as many weeks, although organisers put the number at 550,000.

The anniversary always draws protests but this year's were much larger than normal due to widespread opposition to the extradition bill.

On Tuesday, Chinese state media ran footage of police clearing the protesters from streets in a break with its silence over several past days of pro-democracy demonstrations.

Beijing has largely sought to downplay the demonstrations that have highlighted doubts about the validity of its "one country, two systems" formula for governing the former British colony.

A protester spray paints an emblem. Credit: AP

Its coverage of the protests and the publication of a harsh editorial in the official Communist Party newspaper Global Times may indicate it is prepared to take a tougher line against the demonstrators following days of forbearance.

"These violent assailants in their arrogance pay no heed to Hong Kong's law, no doubt arousing the anger and sadness of all people of the city of Hong Kong," the editorial said.