Hong Kong riot police detained opposition Legislative Councillor Eddie Chu and four district councillors who had been peacefully protesting at a gathering to honour the anniversary of last year's Yuen Long attack in which dozens were injured.
The demonstrators were gathered in the Yoho Mall shopping centre in Yuen Long at about 16:00 local time (08:00 GMT) before the police intervened.
The protesters had planned to march from Yuen Long MTR station, where the attack occurred last year, to Shui Pin Tsuen Playground. However, after being blocked from doing so by riot police the group moved into a shopping centre.
Council Front lawmaker and political activist Chu had been holding a banner and protesting peacefully before police raised a purple flag, signifying a warning that the new national security law was being violated. After verbally warning his group, Chu and two others were detained.
Police went on to detain more protesters, including four district councillors, and also used pepper spray on the demonstrators.
At Yuen Long MTR station last year, an armed mob with wooden sticks and metal poles attacked protesters returning from a rally against the Hong Kong extradition bill. Many civilians were also injured in the attack, which left 47 hospitalised.