Fighting broke out between protesters and a gang of men armed with sticks in North Point on Monday night.
Anti-government protesters had moved into the area and started dismantling railings and water barriers.
Around 8 o'clock, a group of men - many of them wearing white T-shirts - rushed towards the protesters on King's Road, near Ming Yuen Western Street.
A pitched battle followed, with the black-clad demonstrators hurling anything they could find at the assailants.
The attackers eventually fled into a nearby building, which was then besieged by the protesters.