Hong Kong police fired rubber-coated lead bullets, water cannon, and cannisters of tear gas at goggle-wearing demonstrators, who threw plastic bottles and set up metal barricades with umbrellas to protect from pepper spray.
The police commissioner called it a "riot situation".
Clashes continued on Wednesday. Here are the latest updates:
Police defend 'non-lethal' use of force
At a brief news conference held as the chaos swirled just outside, Police Commissioner Stephen Lo Wai-chung called the demonstration a riot.
That could spell long jail terms for anyone arrested, adding to concerns that Hong Kong's government is using public disturbance laws to intimidate political protesters.
"We condemn such irresponsible behavior," Lo said. "There's no need to hurt innocent people to express your opinions," he said, adding that people should not "do anything they will regret for the rest of their lives."
Police spokesman Gong Weng Chun defended the decision to use tear gas and other non-lethal weapons to quell the demonstration.
"I believe that if our officer is not encountering some threat that they probably may suffer from serious bodily harm or even our officer thinks that their life is being threatened, I don't think our officer has any necessity to use any kind of force," he said.
Police officer aims a tear gas gun at protesters during a demonstration against a proposed extradition bill in Hong Kong [Athit Perawongmetha/Reuters]
Protesters tear gassed
Hong Kong police used tear gas, pepper spray and high-pressure water hoses against protesters who laid siege to government buildings to oppose the extradition bill.
"It was a disgrace. Unacceptable. We were all retreating and the police fired a tear gas canister right into the crowd. My eyes started burning, was hard to breathe," Lewis, a 25-year-old said.
Thousands of protesters blocked entry to Hong Kong's government headquarters, delaying a legislative session on proposed changes to the law, which has become a lightning rod for concerns over greater Chinese control and erosion of civil liberties in the semi-autonomous territory.
Frontline of demonstrators in hard hats, their faces covered with masks, moved metal crowd control barriers, ignoring police pleas to stop. After several failed attempts, the crowd gained access to the grounds of the legislature building and the police fired more tear gas to push them back.
After the majority of the crowd was dispersed, about 100 demonstrators could be seen donning gas masks and holding their ground at one section of the complex.
Protesters run away from the tear gas fired by riot police during a rally outside the Legislative Council in Hong Kong [AP]
HK administration: 'Leave the area'
In an earlier statement to reporters, Chief Secretary for Administration Mathew Cheung asked demonstrators to leave the area.
"The Hong Kong government calls on people who are blocking roads to … go back to the pavement as soon as possible," he said.
"I would also like to ask the people in this gathering to stay calm and leave the scene as soon as possible and not to commit any crime."
Cheung gave no indication of when the delayed legislative debate would begin.
China reiterates support for Hong Kong government
China's central government firmly supports the Hong Kong government in passing an extradition law, the foreign ministry reiterated.
Ministry spokesman Geng Shuang told a news briefing in Beijing any actions that harm Hong Kong are opposed by mainstream Hong Kong public opinion. He also urged the United States to speak and act with caution regarding Hong Kong.
Tens of thousands of Hong Kong demonstrators surrounded the Chinese-ruled city's legislature, forcing it to postpone a second round of debate on the bill.
Hong Kong Consulate in Taiwan
Dozens of people protested outside the Hong Kong Consulate in Taiwan to denounce proposed amendments to Hong Kong's extradition laws.
Hong Kong students sitting outside the consulate in Taipei held posters reading "No extradition to China" and chanting: "Hong Kong government, Shame on you."
Ho Wing Tung, a Hong Kong student in Taiwan, said the rule of law won't exist if the legislation passes and she is afraid the "one country, two systems" principle would become a joke.
Hong Kong delays bill debate as crowds mass
Thousands of protesters blocked entry to Hong Kong's government headquarters delaying a legislative session on the proposed law change.
The overwhelmingly young crowd overflowed onto a major downtown road as they overturned barriers and tussled with police outside the building that also houses the chambers where the legislature was to discuss the bill, which would allow criminal suspects in Hong Kong to be sent for trial in mainland China.
A curt government statement said the session scheduled to begin at 11 am would be "changed to a later time." An earlier statement said staff members were advised not to go to work and those already on the premises were told to "stay at their working place until further notice."
Wearing goggles and hard-hats, and protecting themselves with umbrellas, protesters in Hong Kong tried to storm the Legco government complex [Euan McKirdy/Al Jazeera]
Tuesday, June 11
Open debate on controversial bill
Protesters in Hong Kong are gathering outside the semiautonomous Chinese territory's legislature amid plans for further demonstrations and strike actions on Wednesday morning.
The administration of Hong Kong Chief Executive Carrie Lam plans to open debate on the legal amendments on Wednesday.
Largest demonstration in decades
The head of Hong Kong's legislature has announced the schedule for debate on contentious changes to the territory's extradition laws, setting a vote by June 20.
Legislature President Andrew Leung said Tuesday that he had accepted 153 out of 238 proposed amendments to the bills. He said there would be 66 hours for debate.
Hundreds of thousands of people protested against the legislation on Sunday in the largest demonstration in Hong Kong in more than a decade.
The turnout reflected growing apprehension about relations with the Communist Party-ruled mainland.
Hong Kong: Occupy Central
SOURCE: Al Jazeera and news agencies