China's parliament has backed security legislation for Hong Kong which would make it a crime to undermine Beijing's authority in the territory.
The resolution - which now passes to China's senior leadership - has caused deep concern among those who say it could end Hong Kong's unique status.
It could also see China installing its own security agencies in the region for the first time.
The move has already sparked a new wave of anti-mainland protest.
Clashes broke out on Wednesday as Hong Kong's parliament debated a different proposed law, which would make it a crime to disrespect the Chinese national anthem. Hundreds of people were arrested in protests over that and the security law.
Security remains high on Thursday, as a tense debate in the Legislative Council continues.
Full details about exactly what behaviour will be outlawed under the new security law are not yet clear. It is due to be enacted before September.
But hours before the vote was taken in Beijing, US Secretary of State Mike Pompeo said developments in Hong Kong meant it could no longer be considered to have "a high degree of autonomy" from mainland China.
That meant that Hong Kong no longer merited being treated differently from the mainland under US law.
The declaration could have major implications for Hong Kong's trade hub status and drew an angry reaction from Chinese authorities in the territory.
In a strongly worded statement on Thursday, the office of China's foreign ministry in Hong Kong said it "firmly opposed and refuted" Mr Pompeo's statement and urged the US to "immediately stop meddling" in China's internal affairs.
It described US criticism of the new draft law as "utterly imperious, unreasonable and shameless".