Petrol bombs were hurled at a Hong Kong police station and dozens of people have been arrested following a march against so-called parallel trading near the Chinese border.
The Democratic party said about 10,000 people marched peacefully in Sheung Shui district on Sunday, but violence erupted after police ordered protesters to disperse.
Several petrol bombs were thrown at the Sheung Shui police station, about 1.5km (1 mile) from where the rally took place.
The protest comes during a period of heightened anti-mainland sentiment in Hong Kong, where a pro-democracy movement demanding greater freedoms from Beijing has raged for nearly seven months.
The marchers were protesting against parallel trading – thousands of mainlanders cross the border every day to bulk-buy goods such as infant formula to sell at a profit in China.
There is significant resentment against the practice, which frequently leaves goods in short supply in border towns, and has driven up the price of commodities as well as shop rents.
“If the police could spare one of the cars they drove here to handle the march to instead deal with the trading problem, we would not have to organise this protest,” said Dino Chan, a Sheung Shui district councillor and one of the rally organisers.
He added that 42 people were arrested following the violence.
The anti-government protests have been blamed for helping plunge Hong Kong’s economy into recession for the first time in a decade.
The protests were triggered by a proposal to allow extraditions to mainland China, but have morphed into a broader revolt for democratic freedoms.
On Sunday the violence was not at the level seen during many previous protests, with police using pepper spray to disperse crowds but not teargas.
China and the Hong Kong administration have refused to bow to protester demands, which include direct elections, an inquiry into alleged police misconduct and amnesty for the nearly 7,000 people arrested so far.