Hong Kong police fired rubber bullets and tear gas at demonstrators who threw plastic bottles on Wednesday as protests against an extradition bill that would allow people to be sent to mainland China for trial descended into violent chaos.

Tens of thousands of protesters had gathered peacefully outside the Chinese-ruled city's legislature before tempers flared, some charging police with umbrellas.

Police warned them back, saying: “We will use force.”

Ambulances sped towards the protest area as panic spread through the crowd, with many people trying to flee the stinging tear gas, according to a Reuters witness. More than 10 people were wounded in the clashes, Cable TV reported.

Police used pepper spray, tear gas and batons to force the crowds back. Some shops put up their shutters at the nearby IFC, one of Hong Kong’s tallest buildings.

Civil Human Rights Front, which organised a protest on Sunday that it estimated saw more than a million people take to the streets in protest against the extradition bill, accused police of using unnecessary violence.

The protesters, most of them young people dressed in black, had erected barricades as they prepared to hunker down for an extended occupation of the area, in scenes reminiscent of pro-democracy “Occupy” protests that gridlocked the former British colony in 2014.

The violence had died down by early evening under light rain, but tens of thousands still jammed the streets in and around Lung Wo Road, a main east-west artery near the offices of embattled Hong Kong chief executive Carrie Lam.

“Didn’t we say at the end of the Umbrella movement we would be back?” pro-democracy lawmaker Claudia Mo said, referring to the name often used for the 2014 demonstrations, whose trademark was the yellow umbrella.

“Now we are back!” she said as supporters echoed her words.

Others once again called for Lam to step down.

Opposition to the bill on Sunday triggered Hong Kong’s biggest political demonstration since its handover from British to Chinese rule in 1997 under a “once country, two systems” deal guaranteeing it special autonomy, including freedom of assembly, free press and independent judiciary.

But many accuse China of extensive meddling since, including obstruction of democratic reforms, interference with local elections and of being behind the disappearance of five Hong Kong-based booksellers, starting in 2015, who specialised in works critical of Chinese leaders.

Lam has vowed to press ahead with the legislation despite deep concerns in the Asian financial hub, including among business leaders, that it could undermine those freedoms and investor confidence and erode the city’s competitive advantages.

In a brief evening televised address, Lam “strongly condemned” the violence and urged the city to return to normal as soon as possible.

In a separate interview recorded earlier on Wednesday before the worst of the violence, she repeatedly stood by the introduction of the bill, and said the time was right for it to be debated.

“I have never had any guilty conscience because of this matter, I just said the initial intention of our work is still firmly right.”

She added that “perhaps it is impossible to completely eliminate worry, anxiety or controversy”.

The government said debate on the bill that was due to take place in the city’s 70-seat Legislative Council on Wednesday would be delayed until further notice.

The legislature is controlled by a pro-Beijing majority. “We won’t leave till they scrap the law,” said one young man wearing a black mask and gloves.

“Carrie Lam has underestimated us. We won’t let her get away with this.”

Financial markets were hit. The benchmark Hang Seng Index closed 1.7 per cent lower, having lost as much as 2 per cent in the afternoon, while Chinese companies in Hong Kong ended down 1.2 per cent. British Prime Minister Theresa May said extradition rules in Hong Kong had to respect the rights and freedoms set out in the 1984 Sino-British agreement on Hong Kong’s future.

“We are concerned about potential effects of these proposals particularly obviously given the large number of British citizens there are in Hong Kong,” May told parliament.

“But it is vital that those extradition arrangements in Hong Kong are in line with the rights and freedoms that were set down in the Sino-British joint declaration.”

China reiterated its support for the legislation.

“Any actions that harm Hong Kong’s prosperity and stability are opposed by mainstream public opinion in Hong Kong,” Chinese foreign ministry spokesman Geng Shuang told reporters.

Asked about rumours that more Chinese security forces were going to be sent to Hong Kong, Geng said that was “fake news”.

The rally was within sight of the Hong Kong garrison of China’s People’s Liberation Army, whose presence in the city has been one of the most sensitive elements of the 1997 handover.

The protesters, who skipped work, school or university to join the rally, rallied just a stone’s throw from the heart of the financial centre, where glittering skyscrapers house the offices of some of the world’s biggest companies, including HSBC.