Hong Kong's pro-Beijing leader on Monday refused to scrap a controversial plan to allow extraditions to the Chinese mainland, a day after record crowds came out to oppose the proposal.

Striking a defiant tone after the city's largest protest since the 1997 handover, chief executive Carrie Lam said the legislature would debate the bill on Wednesday as planned, rejecting calls to delay or withdraw the law.

The decision sets her administration on a collision course with opponents who called on supporters to rally outside parliament on Wednesday or hold strikes.

"She's really pushing Hong Kong towards the brink of a precipice," pro-democracy lawmaker Claudia Mo told reporters.

Sunday saw huge crowds march through the streets of the financial hub's main island in a noisy, colourful demonstration calling on the government to scrap its planned extradition law.

Organisers said as many as a million people turned out -- the largest protest in three decades and the biggest by far since the city's return to Chinese rule.

Lam's government is pushing a bill through the legislature that would allow extraditions to any jurisdiction with which it does not already have a treaty -- including mainland China.

Authorities say it is needed to plug loopholes and to stop the city being a bolthole for fugitives.

But the proposals have birthed an opposition that unites a wide cross-section of the city, with critics fearing the law will entangle people in China's opaque and politicised courts.

- Incredulous response -

Lam said Monday that the huge rallies proved that Hong Kong's freedom of speech was still protected.

She said her administration had already made concessions to ensure political cases would not be considered and that human rights safeguards met international standards.

"We have been listening and listening very attentively," she said.

Opponents say she is ignoring them.

"Yesterday 1.03 million of us marched and the government is still indifferent, turning a deaf ear to the people. This government has become a dictatorship," lawmaker Ip Kin-yuen told reporters.

Story continues

In Washington, State Department spokeswoman Morgan Ortagus said the overwhelming turnout "clearly shows the public's opposition," and urged Hong Kong leaders to engage a "broad range of local and international stakeholders" on the way forward.

Reiterating concern that the extradition law would erode Hong Kong's special status, Ortagus said she worried that the amendments "could damage Hong Kong's business environment and subject our citizens residing or visiting in Hong Kong to China's capricious judicial system."

Political analyst Dixon Sing warned Lam could be facing "political suicide" if she pushed for a showdown after such huge demonstrations.

"In the short run, the Hong Kong government led by Carrie Lam will suffer a worsening legitimacy crisis," he told AFP. "Fewer and fewer people will trust her and the entire cabinet."

But he said much would rest on whether the public comes out to back further protests or strikes.

Sunday's rally was peaceful until shortly after midnight, when pockets of protesters fought running battles with police in chaotic and violent scenes.

Nineteen people were arrested, police said, mostly men in their twenties.

Hong Kong authorities said they believed the violence was planned by organised groups.

"It's easy to tell they are organised, premeditated, prepared, radical and violent people," said Li Kwai-wah, senior superintendent of the Organized Crime And Triad Bureau.

There was a heavy police presence outside parliament on Monday as officials moved twisted remains of metal barricades and debris left by the overnight skirmishes.

- Years of tumult -

Hong Kong has been shaken by political unrest in recent years as fears soar that a resurgent Beijing is trying to quash the city's unique freedoms and culture.

Under the 50-year handover deal with Britain, China agreed to a "one country, two systems" model where Hong Kong would keep freedom of speech and assembly rights that are unheard of on the authoritarian mainland.

But many locals believe Beijing is now reneging on that deal, aided by the city's loyalist local government.

In 2014 mass protests calling for the right to directly elect Hong Kong's leader paralysed parts of the city for more than two months, with frequent clashes between police and demonstrators.

Two years later violence broke out in the crowded Mongkok district when police tried to close down unlicensed street vendors. Key protest leaders have since been jailed or barred from politics.

Many young Hong Kongers have hardened their attitudes towards China after failing to win any concessions since the 2014 protests.

Chinese foreign ministry spokesman Geng Shuang said Monday that Beijing "will continue to firmly support" the Hong Kong administration, adding that the government will "firmly oppose any outside interference in the legislative affairs" of the city.

In an editorial, Beijing's state-run China Daily called the law a "sensible, legitimate" piece of legislation, and said "some Hong Kong residents have been hoodwinked by the opposition camp and their foreign allies into supporting the anti-extradition campaign".