Hong Kong's Legislative Council delayed the start of a legislative session debating a contentious extradition bill as protesters massed outside to block entry to the chamber and government headquarters.
The Government's press service said the session would be "changed to a later time to be determined" by the secretariat. Council members would be notified of the time of the meeting later.
An overwhelmingly young crowd of demonstrators filled roads in the Wan Chai district.
Many took the day off from work and classes to press their case that the amendments to the extradition bill would erode the semi-autonomous Chinese territory's civil liberties.
Under its "one country, two systems" framework, Hong Kong was guaranteed its own social, legal and political systems for 50 years following its handover from British rule in 1997. However, China's ruling Communist Party has been seen as increasingly reneging on that agreement by forcing through unpopular legal changes.
A vote on the amended laws is scheduled for June 20.
Opponents of legislation that would allow criminal suspects to be extradited to mainland China started gathering outside the Legislative Council early yesterday with further demonstrations and labour strikes expected.
A weekend protest by hundreds of thousands of people was the territory's largest political demonstration in more than a decade.
"We're young but we know that if we don't stand up for our rights, we might lose them," said an 18-year-old protester who gave only her first name, Jacky.
Police called in reinforcements and closed off access to the area around the Legislative Council and government headquarters, patrolling in the heat and humidity.
Jimmy Sham, convener of the Civil Human Rights Front that organised Sunday's protest, said his group is prepared to keep fighting to defeat the legislation.
"We will use our people to surround the Legislative Council," Sham said.
Hong Kong's leader, Chief Executive Carrie Lam, has consistently defended the legislation as necessary to close legal loopholes with other countries and territories.
Sunday's protest was widely seen as reflecting growing apprehension about relations with the Communist Party-ruled mainland, whose leader, Xi Jinping, has said he has zero tolerance for those demanding greater self-rule for Hong Kong.
Critics believe the extradition legislation would put Hong Kong residents at risk of being entrapped in China's judicial system, in which opponents of Communist Party rule have been charged with economic crimes or ill-defined national security offenses, and would not be guaranteed free trials.