Hong Kong police have deployed water cannon against protesters for the first time in 12 weeks of anti-government demonstrations.
The two cannon vehicles were used on Sunday to clear barricades and disperse crowds in the Tsuen Wan district.
As tensions escalated, black-clad protesters threw projectiles, including bricks and petrol bombs.
During one clash, police pointed their guns at protesters. Reports said one officer fired a warning shot.
The demonstrations were sparked by an extradition bill but have since morphed into broader anti-government protests.
Images on social media show the water-cannon vehicles being driven through the streets of Tsuen Wan, where a group of demonstrators had set up roadblocks and dug up bricks from the pavement.
The vehicles are equipped with surveillance cameras and multiple water-cannon nozzles, and police had said they would only be used in the event of a “large-scale public disturbance”.
Earlier this month, human rights group Amnesty International warned that the cannons could cause serious injuries and lead to further tensions.
There was another, more peaceful rally on Sunday comprised of a few hundred people, some of them family members of the police, who called for a political solution to the crisis.
Activists have criticised the police for being heavy-handed during the pro-democracy rallies.
Several people were hospitalised on Saturday after violent clashes.
The latest round of violence comes after an uneasy peace that lasted a little more than a week.
A new level of determination
Saira Asher, BBC News, Hong Kong
For the second day this weekend an industrial neighbourhood in Hong Kong became a battleground.
Parts of Tsuen Wan, on the outskirts of Hong Kong, are known for triad gangs so tensions were high to begin with.
But as a thick fog of tear gas engulfed a major street, it became clear that the brief peaceful lull of last week Hong Kong had enjoyed is well and truly gone.
The extreme group of protesters came ready. Armed with bricks, petrol bombs and metal rods.
And the moderate protesters egged them on from the sidelines shouting “corrupt police” and calling them “piles of rubbish”.
Ordinary residents and spectators were red-eyed from the tear gas and hunching low as they crossed the overhead bridges.
This weekend has been a return to the familiar scenes between the police and protesters, but with what seems like a new level of determination from the protesters.
Why are there protests in Hong Kong?
They were sparked by a now-suspended extradition bill that would have allowed Hong Kong to send criminal suspects to China for trial, but they have grown into a broader movement calling for democratic reform in the territory and an investigation into alleged police brutality against protesters.
Last Sunday, about 1.7 million people attended a pro-democracy rally in central Hong Kong, according to organisers. Police put the figure much lower, at 128,000, counting only those at an officially sanctioned rally in the city’s Victoria Park.
There have been previous protests at Hong Kong International Airport as well as tourist spots in the city.
A guide to the Hong Kong protests