Hong Kong police have fired tear gas and rubber bullets at protesters trying to storm the city's legislature after tens of thousands of demonstrators rallied against a contentious extradition bill.

Key points: Protesters earlier blocked parliament's entry, forcing debate of the bill to be delayed

Protesters earlier blocked parliament's entry, forcing debate of the bill to be delayed Opponents say the bill undermines freedoms guaranteed to the former British colony

Opponents say the bill undermines freedoms guaranteed to the former British colony One million joined protests on Sunday but Hong Kong authorities have vowed to press on

It came after Hong Kong officials called on protesters to leave the area where huge crowds have been surrounding government headquarters, blocking the entrance to the legislative council and delaying a debate on the proposal.

The second reading of the bill that would allow suspects to face trials in mainland China was due Wednesday.

Protesters and police clashed intermittently in running battles outside the city's legislature, as the peaceful rally descended into violent chaos.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 22 seconds 1 m 22 s The clashes came after Hong Kong authorities warned protesters to disperse.

Hong Kong's police commissioner, Stephen Lo Wai-chung, said officers used batons, pepper spray, beanbag rounds, rubber bullets, water hoses and tear gas against the demonstrators after a large group of masked protesters charged on to roads surrounding the headquarters and started throwing objects, including metal barriers, at police.

Earlier, protesters hurled traffic cones and plastic bottles, some charging police with umbrellas.

According to Mr Lo, several people, including police officers, have been injured in the situation, which he described as a riot.

Mr Lo appealed for protesters to leave the area, saying: "This is very dangerous action that could kill someone."

As of 10:00pm on Wednesday, 72 people had been hospitalised, including two classed as serious, according to the Hong Kong Hospital Authority.

An uneasy calm settled on the city overnight as police and protesters braced for potential fresh clashes on Thursday.

Hundreds of riot police could be seen resting and re-grouping overnight while protesters obtained fresh supplies of water, googles and helmets.

Protesters occupy two main highways near the government headquarters in Hong Kong. ( AFP: Philip Fong )

Protesters had been gathered in and around Lung Wo Road, an important east-west artery near the offices of Hong Kong Chief Executive Carrie Lam, for most of the day as hundreds of riot police warned them not to advance.

The protesters, most of them young people dressed in black, erected barricades as they prepared for an extended occupation of the area, in scenes reminiscent of pro-democracy "Occupy" protests that rocked the city in 2014.

"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."

Police officers use water cannons on a lone protester near the government headquarters. ( AFP: Anthony Wallace )

Embattled leader Ms Lam has insisted she would press ahead with the controversial legislation despite deep concerns in the Asian financial hub, including among business leaders, that it could undermine freedoms and investor confidence, and erode its competitive advantages.

Opposition to the bill on Sunday triggered Hong Kong's biggest political demonstration since its handover from British to Chinese rule in 1997.

The bill was due for a second round of debate in Hong Kong's 70-seat Legislative Council, which is controlled by a pro-Beijing majority.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 33 seconds 33 s Many in Hong Kong oppose the proposed law, which would allow people to be sent to mainland China for trial.

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

The legislature is controlled by a pro-Beijing majority.

Human rights groups have repeatedly said the law should not go ahead due to the alleged use of torture, arbitrary detentions and forced confessions in China, where courts are controlled by the Communist Party.

The overwhelmingly young crowd of demonstrators are filling roads in the Wan Chai commercial district. ( ABC News: Brant Cumming )

China has denied accusations that it tramples on human rights.

Ms Lam has sought to soothe public concerns and said her administration was creating additional amendments to the bill, including safeguarding human rights.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 50 seconds 50 s Hong Kong Chief Executive says she would never "sell out" the city

In a tearful interview, said she would never "sell out" Hong Kong and said violence would not be tolerated.

Businesses across the city went on strike on Wednesday in protest against the bill, while teachers and students also walked out and transport go-slows were in place.

A few protesters have been showing a touch of nostalgia for British rule. ( ABC News: Brant Cumming )

HSBC and Standard Chartered banks, in addition to the Big Four accounting firms, had all agreed to flexible work arrangements for staff, Hong Kong media reported.

The Catholic and Protestant churches in Hong Kong both also spoke out against the bill.

In addition to the snowballing backlash, Sunday's protest — which organisers said saw more than a million people take to the streets — could also raise questions about Ms Lam's ability to govern effectively.

Some protesters defied police demands to retreat. ( AP: Vincent Yu )

Many of the protesters defied police calls to retreat and passed provisions, including medical supplies, goggles, water and food among each other.

Opposition politician Hui Chi-fung said he did not expect a repeat of the 2014 protests but highlighted anger in Hong Kong toward what many people in the city see as increasing efforts by Beijing to control the city and stifle its democracy.

"I don't consider today's protest an Occupy movement," he said.

"Young people here in the crowd are here only to voice out their anger at the Government forcing to pass the extradition bill."

The highly controversial extradition law has galvanised Hong Kong residents in an unusually broad show of opposition to the Government. ( AP: Kin Cheung )

In response to the protests, the United Kingdom has urged Hong Kong's Government to "pause and reflect" on the extradition bill.

UK Foreign Secretary Jeremy Hunt urged the former British colony to "listen to the concerns of its people" and the international community.

"It is essential that the authorities engage in meaningful dialogue and take steps to preserve Hong Kong's rights and freedoms and high degree of autonomy, which underpin its international reputation," Mr Hunt said.

Britain handed Hong Kong back to China 22 years ago under a "one-country, two-systems" formula, with guarantees that its autonomy and freedoms, including an independent justice system, would be protected.

However, many accuse China of extensive meddling since then, 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.

Beijing rejects those accusations and Chinese media this week said "foreign forces" were trying to damage China by creating chaos over the extradition bill.

Human rights groups have repeatedly said the law should not go ahead due to the alleged use of torture and forced confessions in China. ( AP: Vincent Yu )

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