Thousands of protesters in Hong Kong took to the streets in Yuen Long, a northern town, despite a police ban.
The march was a response to last Sunday's attack, where about 100 men in white T-shirts descended on Yuen Long's metro station, beating protesters, passersby, and journalists with wooden and metal sticks.
Forty-five people were injured in the attack, which was widely blamed on triad gang members.
Protesters accused the police of turning a blind eye and colluding with the attackers, claims the police deny.
Police say their forces were stretched during another day of unrest, and said suggestions that they had colluded with criminal gangs was a 'smear'.
Police say 12 people have so far been arrested over Sunday's attack, including nine men with links to triads, reports say.
