Thousands of people protested in Hong Kong on Sunday over a proposed law change that would allow suspects to be sent to mainland China for trial.
Officials say they need to amend the law before July so that they can extradite a murder suspect to Taiwan.
Police estimate that about 22,000 people took part, while organisers say this figure was closer to 130,000.
Either way, these estimates would make it the largest demonstration since pro-democracy rallies in 2014 known as the Umbrella Movement.
Reuters news agency reported that people were chanting "step down, Carrie Lam". Ms Lam is the pro-Beijing leader of Hong Kong.
Hong Kong has a separate legal system to mainland China, thanks to "one country, two systems".
The former British colony was handed back to China in 1997 on condition it would retain "a high degree of autonomy, except in foreign and defence affairs" for 50 years.
But earlier this year Ms Lam's government announced that it would overhaul the city's extradition laws so that, for the first time, suspects could be extradited to Taiwan, Macau or mainland China on a case-by-case basis.
Officials said that no-one at risk of being sentenced to death, being tortured, or facing a political charge would be sent to the mainland.
But protesters are unconvinced, with some accusing Ms Lam of having "betrayed" Hong Kong.
They also point to Beijing's poor human rights record and opaque legal system.
Chris Patten, the last British governor of Hong Kong, also told the government-funded broadcaster RTHK the proposal was "an assault on Hong Kong's values, stability and security".