Hong Kong police have fired rubber bullets and tear gas at demonstrators who threw plastic bottles 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."
Police used pepper spray, tear gas and batons to force crowds back, while some individuals were chased.
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.
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.
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.
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.