Embattled Hong Kong leader Carrie Lam has apologised for the way her administration tried to pass a law allowing extraditions to China, the cause of unprecedented protests and clashes this week.

"The chief executive admitted that shortcomings in the government's work has lead to a lot of conflict and disputes in Hong Kong society and has disappointed and distressed many citizens," a statement from her office said.

"The chief executive apologises to the citizens and promises to accept criticism with the most sincere and humble attitude," it added.

Organisers said "almost two million" people turned out for today's mammoth protest in Hong Kong opposed to a deeply unpopular extradition law, an estimate that is nearly double last weekend's already record-breaking crowds.

"Today's march we had almost 2 million people," Jimmy Sham, from the Civil Human Rights Front, told reporters.

Activists set up gazebos as protesters, some carrying white flowers, started to gather in sweltering summer heat to march from Victoria Park to Hong Kong's central government offices.

Beijing-backed Hong Kong Chief Executive Carrie Lam yesterday indefinitely delayed the extradition bill that could send people to mainland China to face trial, expressing "deep sorrow and regret".

The about-face was one of the most significant political turnarounds by the Hong Kong government since Britain returned the territory to China in 1997, and it threw into question Ms Lam's ability to continue to lead the city.

Violent clashes on Wednesday when police fired rubber bullets and tear gas at protesters near the heart of the financial centre grabbed global headlines and forced some banks, including HSBC, to shut branches.

Critics say the planned extradition law could threaten Hong Kong's rule of law and its international reputation as an Asian financial hub. Some Hong Kong tycoons have already started moving personal wealth offshore.

China's Communist Party mouthpiece, the People's Daily, said in a commentary today that central authorities expressed "firm support" for Ms Lam.

The protests have plunged Hong Kong into political crisis, just as months of pro-democracy "Occupy" demonstrations did in 2014, heaping pressure on Ms Lam's administration and her official backers in Beijing.

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The turmoil comes at a difficult time for Beijing, which is already grappling with an escalating US trade war, a faltering economy and tensions in the South China Sea.

The city's independent legal system was guaranteed under laws governing Hong Kong's return from British to Chinese rule 22 years ago, and is seen by business and diplomatic communities as its strong remaining asset amid encroachments from Beijing.

Hong Kong has been governed under a "one country, two systems" formula since its return to Beijing, allowing freedoms not enjoyed on mainland China but not a fully democratic vote.

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

Some opponents of the extradition bill said a suspension was not enough and want it scrapped and Ms Lam to go.

Asked repeatedly on Saturday if she would step down, Ms Lam avoided answering directly and appealed to the public to "give us another chance." She said she had been a civil servant for decades and still had work she wanted to do.

She added that she felt "deep sorrow and regret that the deficiencies in our work and various other factors have stirred up substantial controversies and disputes in society".

China's top newspaper has condemned "anti-China lackeys" of foreign forces in Hong Kong.

"Certain people in Hong Kong have been relying on foreigners or relying on young people to build themselves up, serving as the pawns and lackeys of foreign anti-China forces," the ruling People's Daily said in a commentary.

"This is resolutely opposed by the whole of the Chinese people including the vast majority of Hong Kong compatriots."

The Hong Kong protests have been the largest in the city since crowds came out against the bloody suppression of pro-democracy demonstrations centred around Beijing's Tiananmen Square on 4 June 1989.

Officials said 72 people were admitted to hospitals from the Wednesday protest, while a man died on Saturday after plunging from construction scaffolding where he unfurled a banner denouncing Hong Kong’s extradition bill.

Ms Lam had said the extradition law was necessary to prevent criminals using Hong Kong as a place to hide and that human rights would be protected by the city's court which would decide on the extraditions on a case-by-case basis.