Tensions have flared in the semi-autonomous territory after the ousting of two pro-independence lawmakers ignited fresh concern over Beijing's tightening controls on Hong Kong, which enjoys a separate political and legal system from the mainland.
Britain handed Hong Kong back to China in 1997 under a deal which gives the city broad autonomy and preserves its freedoms and the rule of law for 50 years. But many warn these freedoms are being eroded by Beijing.
In an interview with state broadcaster CCTV late Sunday, the head of China's liaison office in Hong Kong, Zhang Xiaoming, said that controlling the city's independence movement requires strengthening "bottom-line awareness" among its people.
"As for Hong Kong, no one is permitted to engage in any form of activity that harms national sovereignty and security, or challenges the authority of the central government or Hong Kong's Basic Law, or uses Hong Kong to infiltrate and subvert the mainland's social and political stability," Zhang said.
"These are the three bottom lines."
China's Communist Party authorities have viewed the emergence of openly pro-independence politicians with alarm, though only a minority of the city's residents support such a move.
Beijing's increasingly brazen attempts to stifle dissent in Hong Kong have prompted worries over the erosion of its identity as a rules-based business hub - its major draw over mainland rivals such as Shanghai - and a vibrant city with a free press and a distinct way of life.
The disappearance last year of five booksellers known for publishing salacious titles about Chinese political leaders earned international condemnation and realised many residents' worst fears when they resurfaced in detention on the mainland.
Copyright Agence France-Presse, 2017