Disqualified candidates included activist Joshua Wong, some members of the Civic Party and others who won an unofficial primary vote held by the opposition camp earlier this month.
Smaller crowds of protesters in the Mong Kok district set fire to barricades and riot police unleashed 2020's first volleys of tear gas in response.
Police have used tear gas to force hundreds of protesters, some lobbing petrol bombs, to retreat behind make-shift fortifications at the Hong Kong Polytechnic University.
Dozens of protesters were arrested, the youngest aged 12, after police said officers were cornered by demonstrators armed with bricks and other weapons. The firing of a live round elicited an angry response from social media users.
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