After spending hours holding the protesters at bay, police lobbed canisters of tear gas into the crowd this evening. The searing fumes sent protesters fleeing down the road, but many came right back to continue their demonstration.
Students and activists have been camped out on the streets outside the government complex all weekend.
Students started the rally, but by early today leaders of the broader Occupy Central civil disobedience movement said they were joining them to kick-start a long-threatened mass sit-in to demand an election for Hong Kong's leader without Beijing's interference.
The protesters were trying tried to reach a mass sit-in being held outside government headquarters to demand Beijing grant genuine democratic reforms to the former British colony.
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The demonstrations -- which Beijing called "illegal" -- were a rare scene of disorder in the Asian financial hub, and highlighted authorities' inability to get a grip on the public discontent over Beijing's tightening grip on the city.
The protesters reject Beijing's recent decision to restrict voting reforms for the first-ever elections to choose Hong Kong's leader, promised for 2017.
Police officers in a buffer zone manned barricades and doused the protesters with pepper spray carried in backpacks.
The demonstrators, who tried at one point to rip apart metal barricades, carried umbrellas to deflect the spray by the police, who were wearing helmets and respirators.
Police had told those involved in what they also call an illegal gathering to leave the scene as soon as possible, warning that otherwise they would begin to clear the area and make arrests.
To many, it also seemed to mark a major shift for Hong Kong, whose residents have long felt their city stood apart from mainland China thanks to its guaranteed civil liberties and separate legal and financial systems.