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 estimated the crowd at 240,000, but organizers said more than 1 million took part.
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.
Protesters march along a downtown street to against the proposed amendments to an extradition law in Hong Kong Sunday, June 9, 2019.
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, crystalizing worries among critics and others.
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.
The amendments have been widely criticized as eroding the semi-autonomous Chinese territory's judicial independence by making it easier to send criminal suspects to mainland China, where they could face vague national security charges and unfair trials.
Hong Kong's Legislative Council intends to vote on the amended Fugitive Offenders Ordinance and the Mutual Legal Assistance in Criminal Matters Ordinance on Wednesday.
