Police used tear gas and pepper spray near Lan Kwai Fong on Thursday night, as they braced to deal with masked revellers joining the nightclub district's annual Halloween party - and demonstrators protesting against the government's unpopular anti-mask law.
The festival - during which people don masks and scary costumes - has taken on a new significance as Hong Kong government invoked a colonial era ordinance to bring a new law which bans the use of face masks during public events.
The new law also gives police the power to ask a wearer to remove the mask in order to establish the identity of the person.
Police started blocking party-goers from entering Lan Kwai Fong at around 8pm, and later warned people they were taking part in an illegal assembly.
Officers said the area was overcrowded, and they had to take crowd control management measures.
But the party-goers, many wearing masks, refused to leave and locked in a standoff. Some of them shouted at officers that they just wanted to have a drink, and relax.
More police reinforcements arrived, pushing people away from the area and ordering them to remove their masks.
They then used pepper spray and tear gas to disperse the crowd.
Earlier police had set up water-filled barriers and nearby roads were closed. The MTR closed Central MTR station at 9pm.