The Legislative Council is where debates will start on Wednesday to pass new legislation to allow the Hong Kong Government to extradite suspected criminals and fugitives on a case-by-case basis to countries where it does not now have extradition treaties - including mainland China.
Tens of thousands of protesters march along a downtown street against the proposed amendments to an extradition law in HongKong on Sunday, June 9, 2019.
That would make it the biggest rally since a similar number turned out in 2003 to challenge government plans for tighter national security laws, which were later shelved.
They say the laws carry adequate safeguards, including the protection of independent local judges who will hear cases before any approval by the Hong Kong chief executive.
"She has to withdraw the bill and resign", Democratic Party lawmaker James To said to crowds on Sunday night.
"The people of Hong Kong want to protect our freedom, our freedom of speech, our rule of law, our judicial system, and also our economic foundation, which is welcome to worldwide investors", activist Lee Cheuk-yan, a former Hong Kong legislator, said Sunday.
Lam had yet to comment on the rally.
Under the transfer agreement, Hong Kong was supposed to maintain its own political and judicial systems, and for almost three decades, the system existed side by side with the mainland Chinese system, following some of China's basic rules, but preserving basic human freedoms, including freedom from Chinese surveillance and the Chinese justice system, which severely punishes anyone they feel is a threat to the government.
Clashes later erupted between hundreds of demonstrators and police.
Police used batons and fired pepper spray at protesters, who still managed to close off part of a nearby road. The standoff ended in the early hours today.
USA and European officials have issued formal warnings, matching worldwide business and human rights lobbies that fear the changes would dent Hong Kong's rule of law.
Critics of the bill say it would leave anyone on Hong Kong soil vulnerable to being grabbed by the Chinese authorities for political reasons or inadvertent business offenses and undermine the city's semi-autonomous legal system.
"No matter what, no matter they listen to us or not we have to step out, because it is to show not only the Hong Kong government but the people around the world that we have a voice and we disagree with what they are doing", Lam said.
Police had yet to issue their own estimate of the protest size.
Streets were packed along the route.
Some of the protesters, chanting "Scrap the evil law", camped at parks and staged sit-ins outside parliament. He said the bill would have a second reading debate on Wednesday.
Insurance agents, executives and small entrepreneurs joined bus drivers and mechanics in the streets on Sunday.
The crowd included young people and families.
The latest proposal has come after a 19-year-old Hong Kong man allegedly murdered his 20-year-old pregnant girlfriend while they were holidaying in Taiwan together in February last year.
"I need to save my daughter".
"The worldwide community and the business sector are extremely anxious about this law and once you lose confidence in Hong Kong then of course they will just move away their capital". No one will get justice in China. "We will not have the right to express ourselves".
"It puts our legal system on a direct collision course with that of the mainland, when we in Hong Kong talk about the rule of law, traditional independence is a very, very important element", former Hong Kong Bar association chairwoman and senior lawyer Audrey Eu said.
The changes, however, and the speed at which they have been introduced have raised worldwide concern about the future of Hong Kong's legal system and its global reputation. But many of its opponents deeply question the fairness and transparency of the Chinese court system and worry about security forces contriving charges. "Without the sense of security and protection, I think the worldwide city that Hong Kong has always been will be completely changed down the line".
Foreign governments have expressed concern at the law, warning of the impact on Hong Kong's reputation as an worldwide financial hub, and noting that foreigners wanted in China risked becoming snared in Hong Kong.
The men had published lurid tomes about China's leaders and one of them has since fled to Taiwan, saying the risk of extradition is too high.
Jacob Cheng, 62, a sales manager who moved from Hong Kong to Sydney in 1989, said Hong Kong residents had to defend their freedom and democracy for the sake of future generations.