Hong Kong police have fired tear gas at an unauthorised protest involving thousands of demonstrators, report agencies.
Protesters had been marching through the northern district of Yuen Long, condemning an assault on pro-democracy protesters by armed masked men that took place there last week.
Protesters had accused the police of turning a blind eye and colluding with the attackers - claims the police deny.
Hong Kong has seen seven weeks of anti-government and pro-democracy protests.
The protests were sparked by a controversial bill that would have enabled extraditions to mainland China.
The government has since halted the legislation - but protesters are now also demanding an inquiry into police violence, democratic reform, and that the territory's leader Carrie Lam resign.
Saturday's march had been banned by the police - a highly unusual move in the territory - as they said they feared there would be violence.
Despite this, several thousand protesters still converged in Yuen Long, chanting anti-police slogans.
The rally was planned as a response to last Sunday's attack, where about 100 men in white T-shirts descended on Yuen Long's metro station, beating protesters - as well as 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 said the police were slow to respond to emergency calls - and only appeared at the station after the attackers had left.
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.
3 April - Hong Kong government introduces amendments to the city's extradition laws to the legislature that would allow criminal suspects to be extradited to mainland China.
9 June - In the first of many huge protests against the changes, an estimated million people march to government headquarters.
12 June - Anti-extradition bill protesters block roads and try to storm government buildings - police fire tear gas, rubber bullets and bean bag rounds at protesters, in the worst violence the city has seen in decades.
15 June - Hong Kong leader Carrie Lam indefinitely delays the bill in a dramatic reversal.
16 June - Despite this, an estimated two million people take to the streets demanding the complete withdrawal of the bill, an investigation into alleged police violence, and Carrie Lam's resignation.
21 June - As anger grows towards police, protests blockade police headquarters for 15 hours. They now also want protesters that were arrested to be exonerated.
1 July - On the anniversary of Hong Kong's handover from the UK to China, the Legislative Council (LegCo) building is stormed and broken into by protesters.
21 July- Protesters deface China's Liaison Office in Hong Kong. That same night mobs of men wearing white shirts attack protesters and commuters in Yuen Long station, near mainland China, in a new escalation of violence.