Victoria Park, where the protesters gathered, was packed for several hours and became a sea of white - the colour chosen for the action - and smatterings of yellow that came from umbrellas, a pro-democracy symbol since the 2014 movement known as the "Umbrella Revolution", reports Efe news.
The new law, which was proposed in February and the final vote on which is expected sometime in July, would allow the Hong Kong chief executive and courts to process extradition requests of countries with which the former British colony does not have a formal transfer agreement, including mainland China, Taiwan and Macao, without legislative supervision.
In theory, local courts would handle cases individually and could use veto powers to block extraditions, and the Hong Kong executive says the text aims to cover a legal vacuum.
Roads towards the park as well the nearby subway stations were jam-packed with people.
A protester told Efe that should the bill become law, something that could happen next month, Hong Kong would lose its independence to Beijing, and it could affect its local economy negatively.
"It's our last chance to defend our beautiful country," she said, emphasizing the importance of this protest, the first one she has taken part in since June 4, 1989, which was held in solidarity with the students killed on Tiananmen Square in the Chinese capital.
"Beijing has become increasingly impatient with Hong Kong since our umbrella movement five years ago. It sees us as an unruly teenager who doesn't learn to be grateful and obedient," the pro-democracy camp convener, Claudia Mo, told public-owned broadcaster RTHK.
Mo added that with new infrastructure projects, added to laws like the one being protested against on Sunday, Beijing is seeking "to assimilate Hong Kong" and "change it into one of the numerous Chinese cities".
The bill has faced opposition from journalists, foreign politicians and companies, who have expressed concerns that residents of Hong Kong - which is part of China but enjoys special autonomy in fields such as the law and currency - who have been accused of crimes could be at risk of being deported to mainland China.
There were fears that activists and dissidents who live in Hong Kong could be extradited to mainland China for trial.
Demonstrations against the bill have also been held outside of Hong Kong, including in several Australian cities, while more demonstrations were expected to take place in Germany, the US and Canada.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)