In an interview with state news agency Xinhua on Friday, the unidentified spokesman said "some people" ignored the long-term interests of Hong Kong and the provisions of the Basic Law "to collude with external forces in an attempt to interfere [with] the SAR government administration".
This was done "not only to undermine the stability and development of Hong Kong but also to attempt to use Hong Kong as a bridgehead to subvert and infiltrate the mainland", the spokesman said.
"This will absolutely not be tolerated," he said.
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The spokesman did not identify any outside countries. In 2013, U.S. envoy to Hong Kong Clifford Hart said Washington would continue to back "genuine universal suffrage" there.
Beijing's top Hong Kong-based Foreign Ministry representative, Song Zhe, has warned Hart against interference, while Communist Party-backed newspapers have railed against his "subversive" activities.
Such a strongly worded statement from the ministry signals increasing anxiety among Chinese leaders about the outcome of Sunday's decision, which will be watched closely by diplomats and international human rights groups.
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The expected decision to limit the number of candidates for the 2017 elections could trigger a showdown with pro-democracy demonstrators who are planning an "Occupy Central" campaign.
The Hong Kong government will deploy 5,000 police for the expected protest on Sunday night, Hong Kong's official media RTHK said, citing an unidentified source.
Hong Kong returned to Chinese rule in 1997 and there have been fierce debates in the past year over how its next leader is chosen in 2017 - by universal suffrage, as the democrats would like, or from a list of pro-Beijing candidates.