Beijing has warned that protests in Hong Kong are reaching a critical point and threatening the bottom-line of "one country, two systems".
It has also charged that the protesters have become "foot soldiers" of foreign forces bent on destabilising both the city and the mainland.
Both are clear signs that Beijing is treating the demonstrations as a threat to national sovereignty.
The State Council's Hong Kong and Macau Affairs Office (HKMAO) highlighted yesterday the threat to the policy which grants the city a high degree of autonomy, while remaining a part of China.
HKMAO spokesman Yang Guang said radical protesters had made clear their purpose to "make Hong Kong chaotic, paralyse its government, take over the jurisdiction of the Special Administrative Region (SAR) and turn Hong Kong into an independent or semi-independent political entity".
"They want to achieve full autonomy, to confront the central government and eventually make 'one country, two systems' cease to function except in name," said Mr Yang, calling on the Hong Kong government and city residents to safeguard this bottom line.
Fellow spokesman Xu Luying said at the same news briefing that the city was facing its "most grim" situation since its return to China, and that the "most imperative task is to put an end to violence and restore order".
Hong Kong has been convulsed by protests for more than three months, triggered at first by an extradition Bill that would have allowed suspects to be sent to the mainland to stand trial.
The Hong Kong government has suspended the legislation, but protesters want it to be completely withdrawn, and their demands have expanded to, among other things, the resignation of Chief Executive Carrie Lam and universal suffrage.
Beijing has said in the past that the protests show signs of terrorism, and compared them to a "colour revolution".
Mr Yang said yesterday that those who harboured ideas of universal suffrage "would get nowhere".
Ms Xu also said that Beijing would not allow the situation to continue unabated.
"If the situation continues to deteriorate and morphs into turmoil that endangers national sovereignty and security, and is beyond the control of the SAR government, the central government will not sit idly by," she said, adding that People's Liberation Army troops stationed in the city were there to safeguard national sovereignty and security.
One former senior Chinese official told The Straits Times that the Hong Kong government still had options to deal with the protests, including declaring a state of emergency, deploying more police and using more force to put down the unrest.
However, said Professor Wang Zhenmin, former legal chief of the China Liaison Office in Hong Kong, the city's government could not take these measures allowed for under the law if it did not have the "understanding and support" of Hong Kong residents that these were in their long-term interest.
Prof Wang, a law professor at Tsinghua University, added that while Beijing still firmly supported the "one country, two systems" principle, it required everyone to sit down calmly and solve issues at hand.
"If 'one country, two systems' is undermined or cannot work any more, what choice do you have? It is absolutely impossible to return to the past, to return to the United Kingdom. Then, you can only have 'one, country, one system', what would you choose?" he said.