In a clear warning to rising pro-independence forces in Hong Kong, Beijing highlighted its "resolute opposition" to the movement as several young radicals won seats in the weekend's legislative council (LegCo) elections.
The election of the pro-independence candidates and "localists" - who put the interests of Hong Kong above those of the mainland - was unprecedented in a Hong Kong political system that has traditionally been dominated by pro-Beijing and pro-democracy factions, highlighting the disaffection Hong Kongers feel toward Beijing. China pledged a "One Country, Two Systems" policy in Hong Kong after its handover from the U.K. in 1997, a stance some say is fast eroding amid tightening control of political and civil liberties in the city.
In a statement from China's Hong Kong and Macao Affairs Office of the State Council, that was reported by the state-run Xinhua news agency on Monday, the mainland government highlighted that campaigning for Hong Kong's independence was against the constitution of China and the laws of Hong Kong.
The pro-independence movement is a threat to China's sovereignty and security, will damage the prosperity and stability of Hong Kong, and is counter to the fundamental interests of Hong Kong citizens, added Xinhua, citing the office's statement.
Among the localists who will take office on October 1 is 23-year-old Umbrella Movement student leader Nathan Law.
Law will be the youngest legislator in the 70-seat house, which includes 40 pro-establishment and 22 pro-democracy seats. Together, the opposition will make up more than a one-third veto bloc in the first major election since the 2014 protests.
The weekend's polls were the first elections since the Umbrella Movement protests of 2014, which were triggered by proposed changes to Hong Kong's electoral system, under which China planned to screen all nominees in the first direct election of the city's leader.
"What we hope to achieve is to do our best to put a barrier between Hong Kong and the invisible hand of Beijing so that it cannot interfere with Hong Kong's autonomy and freedom," Baggio Leung, one of the eight pro-independence campaigners who won seats, told CNBC.
"But when this barrier cannot hold we will need to separate," the 30-year-old Umbrella Movement alumnus added.
China's Communist administration is highly sensitive to any talk of separatism in regions it considers its territories, including as Xinjiang, Tibet and independently governed Taiwan. Rising calls for independence in Hong Kong will likely heighten sensitivities in Beijing and keep tensions high.
Traditional voices in Hong Kong were already raising concerns about a rift with Beijing.
"Hong Kong's success is really based on the 'One Country, Two Systems' policy; independency is not realistic at all, so hopefully this will not be their only objective in LegCo," veteran lawmaker Elizabeth Quat said of the localist legislators.
While there have been concerns that the group of anti-establishment forces could extend the political paralysis in Hong Kong with their veto power, HSBC's co-head of Asian economic research Fred Neumann said there may be a silver lining to the election result.
"You might actually bring them in and show them how to govern and maybe generate some sort of consensus over what needs to be done...Everybody sitting in the LegCo now has to think very hard about playing ball on certain issues, even if you think some issues are more important. You might see a much more pragmatic approach," he told CNBC's "Squawk Box" on Tuesday.
The LegCo has in recent years been plagued by filibustering, which as stalled the passing of bills.
- Chery Kang contributed to this article.