TENS of thousands of protesters marched through Hong Kong to voice their opposition to government-sponsored legislation that would allow people to be extradited to mainland China to face charges.

The massive protest took place three days before Hong Kong's government plans to bring the bill to the full legislature, bypassing the committee process, in a bid to win approval by the end of the month.

Police said 153,000 people left the protest gathering point at Victoria Park, but that number did not account for those who joined the demonstration along the route. The South China Morning Post said some crowd estimates were as high as 500,000.

Hong Kong's leader, Carrie Lam, has pushed forward with the legislation despite widespread criticism from human rights and business groups. Opponents say that China's legal system would not guarantee the same rights to defendants as in semi-autonomous Hong Kong.

"What can we do to get Carrie Lam to listen to us, how many people have to come out to make her reconsider listening to the public?" said Miu Wong, a 24-year-old office worker who joined the protest.

Hong Kong was guaranteed the right to retain its own social, legal and political systems for 50 years following its handover from British to Chinese rule in 1997, the so-called "one country, two systems" framework. However, China's ruling Communist Party has been seen as increasingly reneging on that agreement by forcing through unpopular legal changes.

Hong Kong currently limits such extraditions to jurisdictions with which it has existing extradition agreements or to others on an individual basis under a law passed before 1997.

China was excluded because of concerns over its poor record on legal independence and human rights. In recent years, mainland authorities have gone after opponents by accusing them of dubious crimes such as tax evasion, crystallising worries among critics and others.

Ms Lam's government argued the revisions were needed to close legal loopholes, while opponents say that is merely an excuse to pursue China's agenda of reducing Hong Kong's legal independence.