Police arrest about a dozen protesters and use pepper spray to break up a protest in Sheung Shui shopping mall aimed at disrupting retail businesses near border with mainland China.
The protests have been largely peaceful for much of December after pro-democracy candidates overwhelmingly won district council elections the month before.
Hundreds of Hong Kong elders have taken to the streets on Saturday to back students for a unity rally as anti-Beijing activists vowed to battle what they say is police brutality and unlawful arrests.
The Polytechnic University stand-off is one of the more dramatic episodes in six months of unrest that began in June with protests over an extradition bill seen by many in Hong Kong as an erosion of freedoms.
Hours before the police operation, a masked protester came out from hiding and told reporters there were fewer than 20 others holed up inside.
Protests against the government switched from violent to peaceful as the city prepared for local elections. Protesters are now weighing their next move as city leader Carrie Lam refuses to compromise.
Candidates looking to loosen China's grip on the semi-autonomous territory grabbed 388 out of 452 seats in 18 district councils, leaving only 59 for the establishment and five for independent candidates.
The first elections in Hong Kong following the months-long pro-democracy protests drew an unprecedented number of voters, with analysts expecting the pro-democracy camp to make gains although still falling short of a majority.
Dozens of students still remain holed up in the besieged Hong Kong Polytechnic University, while the US Senate voted to back Hong Kong.
Police stormed the campus after an all-night siege that included firing repeated barrages of tear gas and water cannons.
Black-clad protesters also occupied university campuses, while office workers endured another day of transport chaos with suspensions on the vandalised train network and roads blocked by barricades.
Police have increased security around Hong Kong and its university campuses as they brace for more violence after sharp clashes overnight with anti-government protesters.
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