Hong Kong police have fired tear gas at protesters angry about an extradition bill that would allow people to be sent to mainland China for trial.
Tens of thousands of demonstrators, most of them young people dressed in black, had gathered peacefully outside government buildings in the Chinese-ruled city before tempers flared, with claims some threw bottles at officers.
Rubber bullets, pepper spray and water hoses were also used to try and keep people back as the protests turned to chaos, with some campaigners charging officers with umbrellas.
Ambulances were seen heading towards the crowds as a police chief warned protesters they "must stop the violence" and urged residents to stay away from the area.
Riot police with shields and batons pushed back against people attempting to storm past barricades to get to into Hong Kong's government headquarters.
Protesters scattered from some areas as claims compressed water was also deployed by officers trying to protect official buildings.
The demonstrations forced the delay of a legislative debate over the controversial bill, which would allow criminal suspects in Hong Kong to be sent for trial in mainland China.
The protests are a challenge to China's ruling Communist Party and President Xi Jinping, who has previously said he would not tolerate Hong Kong being used as a base to challenge the party's authority.
China's central government firmly supports the planned extradition law, the foreign ministry reiterated on Wednesday.
The violence marked a major escalation in the semi-autonomous Chinese territory's biggest political crisis in years.
At a brief news conference, Police Commissioner Stephen Lo Wai-chung called the demonstration a riot - that could mean long jail terms for anyone arrested, adding to concerns that Hong Kong's government is using public disturbance laws to intimidate political protesters.
Mr Lo said: "We condemn such irresponsible behaviour. There's no need to hurt innocent people to express your opinions.
He added that people should not "do anything they will regret for the rest of their lives".
It is understood the police are trying to mass evacuate the flyover in Central, Hong Kong's busy retail and business zone.
Pete Ross, 32, a third generation Hong Kong expat, told Sky News: "It's quite tense here. It's getting quite scary.
"The LegCo (Legislative Council) building is entirely surrounded, so police are trying to clear the roads around it.
"When they try to push a blockade back, people are running at them to prevent them from doing so."
He said police had used tear gas and that despite some protesters wearing protective masks it was "still quite painful".
Crowds overflowed on to a major downtown road as they overturned barriers and tussled with police outside the government building.
Protesters said they hoped the blockade would persuade authorities to shelve the proposed amendments to the extradition bill.
One campaigner, who only gave his name as Marco, to avoid possible repercussions, said: "We want the government to just set the legislation aside and not bring it back."
Another protester claimed the action was a watershed moment for Hong Kong's young generation, saying: "We have to stand up for our rights or they will be taken away."
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