More than 50 people have been injured in Hong Kong protests after hundreds of police officers huddled under riot shields and shot tear gas at demonstrators who stormed and ransacked the city’s parliament.

The Hong Kong hospital authority said 38 men and 16 women were taken to hospital after protesters broke into the Legislative Council late on Monday night to demand a withdrawal of the extradition bill that would allow criminal suspects to be sent to mainland China for trial. 

The South China Morning Post reported that one man and two women were in serious conditions, five men and three women were stable and all others have been discharged.

Dozens of heavily-armed riot police cleared the protesters occupying the legislature after marching into the ground floor of the Legco chamber.

Gathered outside the building were “hundreds upon hundreds of police officers”, CNN reported, in a day of unprecedented chaos and political violence as Beijing marked the anniversary of the city’s 1997 return to Chinese rule.

Police reportedly used a lot of “force and aggression” to clamp down on the thousands of protesters who had spent the entire day in the heart of the former British colony’s financial district.

Protesters ransacked the parliament chambers. Photo: Getty

Police fired several rounds of tear gas as protesters fled or held up umbrellas to protect themselves.

Plumes of smoke billowed across major thoroughfares and in between some of the world’s tallest skyscrapers.

Protesters had carried road signs, others corrugated iron sheets and pieces of scaffolding, as they barged into the council building.

Some sat at legislators’ desks, checking their phones, while others scrawled “anti-extradition” on chamber walls.

Other graffiti called for Hong Kong leader Carrie Lam to step down, while pictures of some lawmakers were defaced.

“HK Is Not China” was painted in black on a white pillar.

The government called for an immediate end to the violence, saying it had stopped all work on extradition bill amendments and that the legislation would automatically lapse in July next year.

The Legislative Council Secretariat released a statement cancelling business for Tuesday.

The central government offices said they would close on Tuesday “owing to security consideration”, while all guided tours to the Legislative Council complex were suspended until further notice.

Riot police in helmets and carrying batons earlier fired pepper spray as the stand-off continued into the sweltering heat of the evening.

Heavily-armed riot police cleared the protestors occupying the legislature. Photo: Getty

The protesters, some with plastic wrap on their arms to protect them from tear gas, once again paralysed parts of the Asian financial hub as they occupied roads after blocking them off with metal barriers.

Ms Lam suspended the extradition bill on June 15 after some of the largest and most violent protests in the city in decades. However, she stopped short of protesters’ demands to scrap it.

The Beijing-backed leader is clinging on to her job at a time of an unprecedented backlash against the government, which poses the greatest popular challenge to Chinese leader Xi Jinping since he came to power in 2012.

Opponents of the extradition bill, which would allow people to be sent to mainland China for trial in courts controlled by the Communist Party, fear it is a threat to Hong Kong’s much-cherished rule of law.

Hong Kong returned to China under a “one country, two systems” formula that allows freedoms not enjoyed in mainland China, including freedom to protest and an independent judiciary.

China has been angered by criticism from Western capitals, including Washington and London, about the legislation.

Beijing said on Monday that Britain had no responsibility for Hong Kong any more and was opposed to its “gesticulating” about the territory.

The EU on Monday called for restraint and dialogue to find a way forward.

Tens of thousands marched in temperatures of about 33 degrees from Victoria Park in an annual rally.

-with AAP