Water cannons were deployed by Hong Kong police against demonstrators on Sunday for the first time in months, after a protest descended into a street battle that left a main thoroughfare littered with bricks and empty canisters of tear gas.

The new police tactic came as violence returned to Hong Kong streets this weekend following a period of relative calm, marked by huge yet peaceful protests. Now in their 12th weekend, the demonstrations are having a significant impact on the financial hub - hurting multinational corporations, deeply unsettling residents, prompting changes in diplomatic procedures and leading to increasingly urgent questions on how this unprecedented period of upheaval will end.

 Police in Hong Kong used tear gas and water cannons to clear out the protesters. Photo / AP
Police in Hong Kong used tear gas and water cannons to clear out the protesters. Photo / AP

Sunday's authorised march in Tseun Wan, in the western part of the New Territories area which borders mainland China, turned rowdy as protesters blocked streets and built barriers to hold back police.

At a point later in the night, a police officer fired a warning shot from his revolver into the air, after protesters armed with metal rods tried to charge at the officers.

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Riot police quickly announced they would begin a dispersal operation, an emerging tactic as authorities grow more and more intolerant of the dissent that continues to grip the city.

Riot policemen march in front of water cannon trucks after clearing demonstrators on a Hong Kong street. Photo / AP
Riot policemen march in front of water cannon trucks after clearing demonstrators on a Hong Kong street. Photo / AP

After volleys of bricks were thrown at police, they responded with rounds of tear gas, and brought forward the water cannons acquired in 2018. They were briefly fired against protesters, sending a group retreating into a nearby mall.

"I do worry about [the water cannons]" said one 25-year-old protester who spoke on the condition of anonymity, citing fear of arrest. "And I can see lots of people on the scene, they do share the same fear with me."

He added that front line demonstrators like him are "still learning," and had to be a bit "conservative" in dealing with the water cannons, which they saw Sunday for the first time.

"Maybe we retreat first and try to figure out how to tackle it later," he said.

Demonstrators damage a steamboat restaurant run by pro-China's owner during a protest in Hong Kong on Sunday. Photo / AP
Demonstrators damage a steamboat restaurant run by pro-China's owner during a protest in Hong Kong on Sunday. Photo / AP

Rights groups have warned that water cannons are "inherently indiscriminate" especially in Hong Kong's crowded streets and residential neighborhoods, but security experts believe the risk of collateral damage is lower than tear gas, which has filled neighborhoods repeatedly over past weeks and seeped into apartments.

Authorities over the weekend closed subway stops in and around neighborhoods where protests were planned, widely seen as a move to limit participation. The MTR Corporation, which runs the subway, has come under fire from Chinese state press which has accused the transit authority of facilitating the protests and labeled them an "accomplice to rioters."

In a service announcement Sunday, the MTR Corporation said the closure was a "prudent measure" to "ensure the safety of passengers and our staff."

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Police arrested 29 people on Saturday between the ages of 17 to 52 "for offences including unlawful assembly, possession of offensive weapon and assaulting police officers," the force said in a statement. There were no immediate reports of arrests on Sunday, though riot police were seen pinning down protesters in Tseun Wan and cuffing them.

Riot policemen fire weapons during a confrontation with demonstrators. Photo / AP
Riot policemen fire weapons during a confrontation with demonstrators. Photo / AP

The latest arrests mean almost 800 people have been detained during the demonstrations, which started in early June over a now-suspended bill that would have allowed extraditions to mainland China.

Concerns over the bill are no longer at the forefront of protesters mind. Instead, they say, they are primarily fighting to keep Hong Kong's special status and to win democracy for their city in the face of an assertive, intolerant Beijing.

Hong Kong leader Carrie Lam on Saturday urged protesters to "sit down and talk" and aims to establish dialogue with the city's youth.