HONG KONG — Police turned ­water cannons against demonstrators on Sunday for the first time in months, as a protest descended into a street battle that left a main thoroughfare littered with bricks and empty canisters of tear gas.

Violence returned to Hong Kong streets over the weekend after a period of relative calm, marked by huge yet peaceful protests. The demonstrations, now in their 12th weekend, are having a significant impact on the financial hub — hurting multinational corporations, deeply unsettling residents, prompting changes in diplomatic procedures and raising increasingly urgent questions about how this upheaval will end. 

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

A police officer fired a warning shot into the air from his revolver after protesters armed with metal rods tried to charge at the officers.

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

Bricks were thrown at police, who responded with rounds of tear gas and deployed water cannons acquired last year. They turned the cannons briefly 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 for the first time on Sunday. 

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

Rights groups have warned that water cannons are “inherently indiscriminate,” especially in Hong Kong’s crowded streets and residential neighborhoods. But security analysts say the risk of collateral damage is lower than with tear gas, which has filled neighborhoods repeatedly over the past weeks and seeped into apartments. 

Authorities closed subway stops in and around neighborhoods where protests were planned, a move widely seen as an effort to limit participation. The MTR Corp., which runs the subway, has come under fire from Chinese state media, which has accused the transit authority of facilitating the protests and called it an “accomplice to rioters.” 

In a service announcement Sunday, MTR called the closure a “prudent measure” to “ensure the safety of passengers and our staff.”

Police arrested 86 people over the weekend, the youngest of them aged 12, on a range of charges including unlawful assembly, assault of an officer and possession of offensive weapons.

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

The bill is no longer the protesters’ main concern. Instead, they say, they are fighting primarily to keep Hong Kong’s special status and to win democracy for the 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 young people. 

Timothy McLaughlin contributed to this report.