Earlier, police baton-charged a crowd blocking a key road in the government district in defiance of official warnings that the demonstrations were illegal.
Several scuffles broke out between police in helmets, gas masks and riot gear, and demonstrators angered by the tear gas, last used in Hong Kong in 2005.
The unrest is the worst since China took back control of Hong Kong from Britain in 1997. It poses a serious challenge to Communist Party leaders in Beijing, concerned that calls for democracy could spread to cities on the mainland and threaten their grip on power.
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