"Foreign forces" are trying to hurt China by creating chaos in Hong Kong over an extradition bill that has prompted mass protests in the former British colony, an official Chinese newspaper said on Monday.
"The people's voices are not being heard", 18-year-old student Ivan Wong told AFP news agency.
Human rights groups have said that China's justice system has a record of arbitrary detention, torture and violations of fair trial rights.
Shortly before the demonstration, the Hong Kong government said in a statement that the measure would not "in any way impact on, interfere with, or have a chilling effect on the freedom of assembly, of the press, of speech, of academic freedom or publication; or relate to offenses of a political nature".
Hundreds of thousands of people, dressed in white, are marching through Hong Kong, in Asia, in its biggest protest in years. Critics say the proposed law would allow mainland China to pursue its political opponents in the city, which has traditionally been a haven from the Communist party. Water refill stations have been set up by volunteers, too.
Some carried yellow umbrellas, a symbol of the 2014 pro-democracy Occupy protests that choked city streets for 79 days.
BBC and CNN reports on the protest were blanked out in China, although the channels can only be viewed in high-end hotels and a small number of apartment buildings and are not available to most Chinese.
Concern about the amendments has spiraled in recent weeks, taking in pro-business and pro-Beijing elements usually loath to publicly contradict the Hong Kong or Chinese governments.
But the proposal has gridlocked the city's Legislative Council, which is roughly divided between pro-democratic and pro-Beijing camps.
"It may be useless, no matter how many people are here".
What are the proposed changes?
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The requests will then be decided on a case-by-case basis.
"It's a proposal, or a set of proposals, which strike a awful blow ... against the rule of law, against Hong Kong's stability and security, against Hong Kong's position as a great worldwide trading hub", the territory's last British governor, Chris Patten, said on Thursday.
Large-scale protests in 2012 and 2014 were largely spearheaded by students, but the extradition bill has also grabbed the attention of Hong Kong's business community, which fears it would weaken the rule of law essential to the success of the worldwide finance center.
A police officer swings his baton as he restrains a protester during the clear up after the clash outside the Legislative Council in Hong Kong after a rally against a controversial extradition law proposal in Hong Kong.
Hong Kong government officials have repeatedly defended the plans, even as they raised the threshold of extraditable offences to crimes carrying penalties of seven years or more.
Officials initially seized on the murder past year of a young Hong Kong woman holidaying in Taiwan to justify swift changes.
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.
When Hong Kong was transferred to China in 1997 under the "one country, two systems" deal, the territory was guaranteed a high degree of autonomy, allowing it to keep its own political, legal and economic systems until 2047.
China was handed back the city on the terms that it would be a semi-autonomous region, governed under the principle of "one country, two systems".
Such a move would be in direct contravention of the conditions mandated when the United Kingdom handed its former colony back to communist China in 1997, Reuters reports.
Critics have attributed such failures to poor legal protection for defendants under Chinese law.