Pro-democracy activists say they fear “the end of Hong Kong”, after China announced plans for a new security law.
The US said the move could be “highly destabilising” and undermine China’s obligations on Hong Kong’s autonomy.
China’s National People’s Congress will on Friday debate the law, aimed at banning sedition and subversion.
Supporters say it is needed to tackle the violence in political protests that erupted last year. Opponents fear it will be used to remove basic freedoms.
Hong Kong has observed a “one country, two systems” policy and a “high degree of autonomy” since Britain returned sovereignty to China in 1997.
But activists, and the pro-democracy movement, feel that this is being undermined by Beijing.
Last year, millions took to the streets over seven months to protest against a bill that would have allowed extraditions to mainland China. Many of the protests turned violent. The bill was eventually paused, and then withdrawn.
The security law is more controversial still. According to the Basic Law, the territory’s mini-constitution, Hong Kong’s government is required to pass national security legislation. However, an attempt in 2003 failed after 500,000 people took to the streets in opposition.
That is why an attempt now to force through national security legislation – which one legislator on Thursday called “the most controversial [issue] in Hong Kong since the handover” – has caused such outrage.
The BBC’s China correspondent, Robin Brant, says that what makes the situation so incendiary is that Beijing can simply bypass Hong Kong’s elected legislators and impose the changes.
China can place them into Annex III of the Basic Law, which covers national laws that must then be implemented in Hong Kong – either by legislation, or decree.
Pro-democracy activists fear the law will be used to muzzle protests in defiance of the freedoms enshrined in the Basic Law, as similar laws in China are used to silence opposition to the Communist Party.