Hong Kong pro-democracy MPs have disrupted a top Beijing official's speech as he sought to explain a decision to curb voting reforms for the southern Chinese financial hub.

They chanted slogans and held up placards accusing China's central government of "breaking its promise" to let Hong Kong directly elect its leader.

The noisy demonstration at the start of Li Fei's address was a rare occasion on which a Beijing official faced open defiance in the former British colony.

Mr Li, a deputy secretary general of the Standing Committee of the National People's Congress, continued his speech after security officers hustled the protesting politicians out of the auditorium.

Beijing inflamed political tension yesterday by ruling out open nominations of candidates running for Hong Kong's top job in inaugural elections in 2017.

Police used pepper spray on members of a radical activist group attempting to storm metal barricades and enter the venue.

The widely-expected voting rules announcement sets the stage for escalating confrontations between China's central government and democracy supporters in Hong Kong who have pledged to carry out a civil disobedience campaign that could culminate in a mass protest to cripple the city's financial district.

Beijing has pledged to allow voters, rather than an elite committee of mostly pro-Beijing tycoons, to elect Hong Kong's leader in 2017.

But it wants to limit candidates to two or three who must be approved by a nominating body similar to that committee, raising fears that candidates will be screened for loyalty to Beijing.