China has strongly condemned protesters who stormed Hong Kong's Parliament on Monday night, while warning other countries to stay out of its internal affairs.

In its toughest response since protests over a controversial extradition Bill erupted last month, Beijing yesterday labelled those who broke into the Legislative Council (LegCo) building as "extremists", and called their act a "blatant challenge to the 'one country, two systems' " model that governs the city.

It issued a statement expressing "resolute support" for the Hong Kong government and police for ending the dramatic takeover and their criminal investigation of the demonstrators.

A spokesman for the Hong Kong and Macau Affairs Office said July 1 was meant to be a celebratory day to commemorate the 22nd year of Hong Kong's return to China.

"Yet, some extremists on the pretext of opposing the amendments to the relevant Bill... attacked the Legislative Council building in an extremely violent manner and deliberately damaged its facilities," the spokesman said a statement.

"This serious illegal act tramples on the rule of law in Hong Kong, undermines Hong Kong's social order and undermines the fundamental interests of Hong Kong."

The central government has so far acted with restraint.

Opponents saw the Bill as an attempt by China to tighten its grip on the city and control dissent as it would potentially allow Beijing to seize anyone in Hong Kong to face the law on the mainland.

At a 4am news conference yesterday after police reclaimed the LegCo premises, the city's Chief Executive Carrie Lam described the coup as "shocking and saddening", and vowed to "pursue the lawbreaking behaviour to the end".

Rejecting accusations that the government had not responded to the demands of the protesters, Mrs Lam said: "The Bill will expire, or the Bill will die, in July 2020 when the current LegCo term expires. That is a very positive response to the demands we have heard."

Mainstream pro-democracy supporters were quick to point out that the hundreds of protesters who stormed the LegCo building did not represent them.

Calm returned to the city yesterday, but the LegCo complex was sealed off by police. All meetings at the building for the next two weeks have been cancelled.

China had earlier blamed "foreign forces" for the mass street protests. Yesterday, it lashed out at the United States, Europe and Britain for their "rude interference" in China's domestic issues, warning them not to support the "violent offenders".

"Don't give out any misleading signals, and don't make any wrong moves," spokesman Geng Shuang said at a regular news briefing.

US President Donald Trump, British Foreign Secretary Jeremy Hunt and the European Union have spoken up for the protesters, drawing the ire of Beijing, which has slammed their remarks as "hypocritical" and "irresponsible".

While Chinese censors have been scrubbing mention of the protests on the mainland for weeks now, Beijing eased control on news coverage of the protests after the turn of events on Monday night, with state media giving coverage to Mrs Lam's pre-dawn briefing and producing strongly worded editorials condemning the violence.

Nationalistic tabloid Global Times called for "zero tolerance" in an editorial yesterday, saying it was "the only remedy for such destructive behaviour". "Out of blind arrogance and rage, protesters showed a complete disregard for law and order," it said.

Hong Kong residents' fear of an erosion of their autonomy has manifested in protests over the years, notably the Umbrella Movement campaign in 2014.

Yesterday, some Chinese netizens, responding to the news reports, called for tough justice.

"It is a group of low-level types taking the opportunity to make trouble... They just want to take the opportunity to vent," said one on Chinese blogging site Weibo.

But some questioned the "one country, two systems" policy.

" 'One country, two systems' is too lax, and this is the result. If Hong Kong wants to return to normal, it should start with decolonisation and change the name of Victoria Harbour to Oriental Pearl Harbour," said another netizen.