The director of Beijing's Liaison Office in Hong Kong says the formation of a political party advocating Hong Kong's independence has exceeded free speech.
Zhang Xiaoming said the matter had touched on the bottom line of the One Country, Two Systems principle, and cannot be tolerated.
Earlier this week, the Hong Kong National Party announced its formation. The party wants Hong Kong to be an independent republic.
Zhang was speaking in an interview with Phoenix Television.
Meanwhile the former Chief Executive, Tung Chee-hwa, has said he hopes more people will speak up in opposition to Hong Kong's independence.
He described the idea as "very naive" and not something properly thought through.
Tung, who's now the vice chairman of China's top advisory body, said independence has absolutely no benefits for Hong Kong.
The discussion of Hong Kong independence, however, is legal, according to a leading legal academic.
Speaking on RTHK’s Newswrap programme, Simon Young, a law professor at the University of Hong Kong, said simply discussing Hong Kong independence is not against the Basic Law.
“We have to be clear. The Basic Law itself does not prohibit this kind of speech,” he said. “In fact because of its protection of freedom of expression one would think it sanctions just speech in that regard.”
Young said the discussion would have to be accompanied by violence for it to be considered an offence.
“I think where things become of more concern is when the speech is accompanied by actions that may be violent or unlawful.”
Beijing has praised SAR officials for refusing to register the Hong Kong National Party as a company.
Hong Kong has no law governing the establishment of political parties. They are usually formed by setting up a company.