Chaos has erupted on the streets of Hong Kong as police clashed with pro-democracy campaigners after more than one million people were estimated to have marched through the city to protest against a proposed extradition deal.

Key points: The amended law would allow case-by-case transfers of people to countries without extradition treaties, including China

The amended law would allow case-by-case transfers of people to countries without extradition treaties, including China Debates will start on Wednesday to pass the legislation

Debates will start on Wednesday to pass the legislation But some are concerned the legal profession will have little power to fight against an extradition request once the laws are passed

Several hundred riot police armed with batons, shields, tear gas guns and pepper spray sealed off the Legislative Council as a similar number of protesters charged their lines shortly after midnight.

Police used batons and fired pepper spray at protesters, who still managed to close off part of a nearby road. Several people on both sides appeared to be injured and ambulances were called. Metal barriers were left twisted and torn in the clashes.

Protests began to turn violent as night fell. ( Supplied )

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The Legislative Council is where debates will start on Wednesday to pass new legislation to allow the Hong Kong Government to extradite suspected criminals and fugitives on a case-by-case basis to countries where it does not currently have extradition treaties — including mainland China.

Earlier on Sunday, hundreds of thousands had jammed Hong Kong's streets to protest against the bill in the biggest demonstration in years. Many said they feared it put the city's vaunted legal independence at risk.

The older generation of Hong Kong protesters left earlier in the evening while a younger crowd made up of university students and young professionals continued to rally.

The rallies — and the violence — plunged the global financial hub into a fresh political crisis, with marchers and opposition leaders demanding the bill be shelved and that the city's Beijing-backed chief executive Carrie Lam resign.

Riot police on the streets of Hong Kong. ( ABC News: Kathryn Diss )

After seven hours of marching, organisers estimated 1,030,000 people took part, far outstripping a demonstration in 2003 when half that number hit the streets to successfully challenge government plans for tighter national security laws.

A police spokesman said police estimated 240,000 were on the march "at its peak".

"She [Lam] has to withdraw the bill and resign," veteran Democratic Party MP James To told crowds gathering outside the council and government headquarters in the Admiralty business district on Sunday night.

"The whole of Hong Kong is against her."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 3 seconds 1 m 3 s Hundreds of thousands protest against Hong Kong extradition laws (Reuters)

Ms Lam told reporters her government would go ahead with proposed amendments despite the protests, saying the legislation would help Hong Kong uphold justice and fulfil its international obligations.

Safeguards added in May would ensure the legislation protected human rights, she said.

Ms Lam denied she was taking orders from the central government in China's capital.

"I have not received any instruction or mandate from Beijing to do this bill," she said.

"We were doing it — and we are still doing it — out of our clear conscience and our commitment to Hong Kong."

Ms Lam said security forces would be back on the streets today, with more protests planned for this evening.

The legislation could be passed into law by the end of June.

State-owned news outlet China Daily editorialised that "foreign forces" were trying to cause chaos and argued the law was much needed.

Protesters march along a downtown street to demonstrate against the proposed amendments. ( AP: Vincent Yu )

"Any fair-minded person would deem the amendment bill a legitimate, sensible and reasonable piece of legislation that would strengthen Hong Kong's rule of law and deliver justice," the editorial said.

"Unfortunately, some Hong Kong residents have been hoodwinked by the opposition camp and their foreign allies into supporting the anti-extradition campaign."

Hong Kong resident Chapman Chen joined the protest. ( ABC News: Kathryn Diss )

The case that sparked furore

A 20-year-old man, Chan Tong-Kai, was at the centre of the firestorm around the bill.

Ms Lam proposed the legislation earlier this year after Taiwan requested his extradition to face charges of allegedly murdering his pregnant girlfriend while on holiday there.

In April, he was convicted by a Hong Kong court of money laundering — in relation to his girlfriend's murder — but authorities were unable to charge him with murder in local courts.

With Chan's release is possible as early as October having already served time, the Hong Kong Government is using the case to push for the extradition law amendment to pass by next month.

Protests against Hong Kong extradition laws. ( ABC News: Amy Ip )

The demonstration capped weeks of growing outrage in the business, diplomatic and legal communities, and human rights groups, which fear corrosion of Hong Kong's rule of law and the lack of an fair and open legal system on the mainland.

Chants of "no China extradition, no evil law" echoed through the streets of the high-rise city as marchers snaked through the Causeway Bay and Wanchai shopping districts.

Some carried yellow umbrellas, a symbol of the 2014 pro-democracy Occupy protests that choked city streets for 79 days.

"It puts our legal system on a direct collision course with that of the mainland, when we in Hong Kong talk about the rule of law, traditional independence is a very, very important element", former Hong Kong Bar association chairwoman and senior lawyer Audrey Eu said.

Audrey Eu is the former Hong Kong Bar association chairwoman and a senior lawyer. ( ABC News: Kathryn Diss )

"But in the PRC (Peoples Republic of China), Xi Jinping, the President and also the head of the supreme people's court have said expressly, judicial independence is a mistaken Western concept.

Ms Eu said the legal profession would have little power to fight against an extradition request once the laws had passed, and warned it could see foreigners living, working or transiting through Hong Kong also within reach of Chinese laws.

"Once the law has passed this will have repercussions not just for Hong Kong residents," she said.

"The international community and the business sector are extremely worried about this law and once you lose confidence in Hong Kong then of course they will just move away their capital."

The proposed extradition law changes have reignited the democracy movement. ( Reuters: Tyrone Siu )

Amendment will 'take away the firewall'

When Hong Kong was transferred to China in 1997 under the "one country, two systems" deal, the territory was guaranteed a high degree of autonomy, allowing it to keep its own political, legal and economic systems until 2047.

"The whole essence of one country two systems is you have two separate systems, in particular the legal system," Ms Eu said.

"What this amendment is going to do is take away the firewall, take away the protection between the two different legal systems."

But the Government argues the laws are necessary to fight crime, including drug manufacturing and trafficking, money laundering and firearms smuggling.

Demonstrators took to the streets to demand authorities scrap the proposal. ( Reuters: Tyrone Siu )

Protests against extradition law spill into Sydney

A group of 1,000 protesters gathered in Sydney on Sunday to urge the Australian Government to condemn the proposed new law.

Accountant Ida Lee said she valued her freedom of speech, and that expatriates feared being seized by China as they travelled through Hong Kong.

"Ordinary people like me, I think, will live in perpetual fear of breaking some law in China, and as we're passing through Hong Kong we'll be arrested and extradited," she said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 39 seconds 39 s Protesters gather in Sydney to condemn Hong Kong extradition law (Image: Reuters)

Protest organiser Jared Fu, a university student, called on Australia to follow the United States, Canada and the European Union in condemning the law.

"Our major concerns regarding this bill include possible political persecution and human rights violations and even threats to personal safety if detained by China," he said.

A Department of Foreign Affairs and Trade spokesperson said Australia's consul-general in Hong Kong had raised the issue with senior levels of the city's government.

Protesters fear the laws will put basic human rights at risk. ( Reuters: Tyrone Siu )

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