Scuffles broke out on Monday during a tense stand-off at the entrance to a center where a senior Chinese official was explaining Beijing's decision, prompting police to use pepper spray amid chaotic scenes inside and outside the venue.
Activists from a movement called Occupy Central have threatened to lock down Hong Kong's financial district on an unspecified date unless Beijing grants full democracy.
"Occupy Central is an illegal activity. If we give in, it will trigger more illegal activities," said Li Fei, deputy secretary general of the NPC Standing Committee, who flew to Hong Kong to explain Beijing's decision.
Pro-democracy activists inside the building heckled Li, shouting slogans and interrupting his speech.
Hong Kong returned to Chinese rule in 1997 with wide-ranging autonomy and freedoms not enjoyed on the mainland under a policy of "one country, two systems".
The activists want universal suffrage, but Communist Party rulers in Beijing say any candidate for the territory's chief executive has to be first approved by a nominating panel - likely to be stacked with pro-Beijing loyalists and making it almost impossible for an opposition democrat to get on the ballot.
Charles Rivkin, U.S. Assistant Secretary of State for Economic and Business Affairs, said in Singapore the United States backed "free and fair elections and transparency".
"We believe, in the case of Hong Kong, in one country and two systems," he said.
British Deputy Prime Minister Nick Clegg said in July that Britain "would not shy away from defending" one country, two systems.
Britain made no mention of democracy for Hong Kong until the dying days of about 150 years of colonial rule.
Disruptive Hong Kong protests loom after China rules out democracy | Reuters