Hundreds of thousands protest new laws that allow the extradition of convicts to other Chinese territories.
Huge crowds spilled out onto the streets of Hong Kong to protest a new law, which would allow the extradition of residents to other Chinese territories.
The unrest is the largest since the British handover of the island to China in 1997.
Organisers of Sunday’s protest put the figure at almost one million people, or roughly a seventh of the population, while police said the number was 240,000 at most.
Protesters chanted "Scrap the evil law!" and "Oppose China extradition!"
The law allows authorities to move suspected criminals to the Chinese mainland for trial.
Critics have raised fears that the move would lead to arbitrary detentions, political persecution, and the threat of torture under the Chinese judicial system.
Hong Kong's pro-Beijing leader Carrie Lam said that she had no plans to scrap the controversial plan despite the outrage.
"This is a very important piece of legislation that will help to uphold justice and also ensure that Hong Kong will fulfil her international obligations in terms of cross-boundary and transnational crimes," she said.
What is the proposal?
The Fugitive Offenders and Mutual Legal Assistance in Criminal Matters Legislation (Amendment) Bill 2019 (the Bill) proposes to allow the extradition of criminals who commit crimes, such as murder and rape, if requested to do so by mainland China, Taiwan and Macau.
However, the extradition will be decided case-by-case.
According to Hong Kong officials, the court will not permit extradition on the basis of political and religious beliefs.
In February last year, a Hong Kong man, aged 19, allegedly murdered his 20-year-old girlfriend during a holiday in Taiwan. He then fled to Hong Kong.
The decision to introduce the extradition law came after the incident and the Hong Kong government is basing the law on a report into the 2018 killing.
“(The law) seeks to plug the loopholes in Hong Kong's overall co-operation mechanism in criminal and juridical assistance matters, including the geographical restrictions and impractical operational requirements under the existing laws,” the report said.
Hong Kong’s current legal status with in China is "one country, two systems". After the handover, Hong Kong became a part of China but maintains its own political and legal systems.