Police fired tear gas and protesters threw bottles and rocks during clashes at a banned rally in Hong Kong.
Several thousand pro-democracy demonstrators marched to protest against suspected gang attacks that left 45 people injured last weekend.
Many in the pro-democracy movement accuse the police of reacting slowly to Sunday’s violence, while others blame local villagers they suspect of having close links to triad groups.
Many carried umbrellas and chanted anti-police slogans, as they rallied, despite police having banned the march on public safety grounds.
After a tense stand-off in Yuen Long district on Saturday afternoon, riot police resorted to tear gas after crowds ignored calls to leave.
There are unconfirmed reports of injuries.
As it grew dark, police eventually pushed activists into retreat.
Sky’s Asia correspondent, Tom Cheshire, said the situation was “utter chaos.”
Police, he said, “pushed protesters back and the protesters in turn pushed them back”.
There was “a lot of anger on the streets here today”, he added, noting that protesters were “a lot more organised, wearing protective hats and masks”.
Triads are organised criminal networks known as the Chinese mafia. They usually run protection rackets, prostitution and low-level drug supply.
Last Sunday, 100 white shirted men stormed the Yuen Long station hours after protesters marched through central Hong Kong and defaced China’s Liaison Office – the leading symbol of Beijing’s authority over the former British colony.
They attacked black-clad protesters returning from Hong Kong island, passers-by, journalists and lawmakers with pipes and clubs, leaving 45 people injured.
Cico Lau, 25, whose brother was among those beaten last Sunday, said: “I come out as I want to express my dissatisfaction with police. What had happened last week was totally unacceptable.”
The demonstrations were initially triggered by a bill, since abandoned, that would allow extraditions to mainland China.
But they have become a referendum on the Hong Kong government and the eroding of the city’s freedoms under Beijing’s control.
Protesters are also demanding independent inquiries into police use of force, the resignation of Hong Kong leader Carrie Lam and full democratic reform.
On Friday thousands of activists gathered in the arrivals halls of Hong Kong international airport.
In a sign of the growing tension in government, police are furious Hong Kong’s chief secretary apologised for last weekend’s attacks apparently without consulting them.
Matthew Cheung said the government would not shirk its responsibility “and the police’s handling fell short of residents’ expectations”.
Britain handed Hong Kong over to Chinese rule in 1997 amid guarantees that its core freedoms and autonomy, including the right to protest and an independent judiciary, would be protected under a “one country, two systems” formula.