A Hong Kong police officer fired a live shot into the air as protesters squared off against police armed with water cannons on Sunday, in one of the most violent nights seen since mass pro-democracy protests began 12 weekends ago.

Commentary in Chinese state media on Sunday signalled that Beijing is losing patience with protesters.

State-run media outlet Xinhua said the central government had the “authority” and “responsibility” to intervene and prevent riots in Hong Kong, reiterating comments made by former Chinese leader Deng Xiaoping, who oversaw Hong Kong's return to China from British rule under a dual governance setup, which gives the city a separate legal system with more democratic freedoms until 2047.

Sunday's pro-democracy protests had begun peacefully. Thousands braved a thunderstorm to march from Kwai Chung to Tseun Wan district, in the city's New Territories. They showed up dressed in plastic ponchos and raincoats, and huddled under a sea of umbrellas.

But the violence returned on Sunday evening, when a smaller, more hardcore group of protesters broke away from the day's main police-approved march – now a familiar feature of the protests. They used traffic cones and street railings to build makeshift barricades in the street, and threw bricks and petrol bombs at police. Fifteen police officers were injured and sent to the hospital for treatment, officials said Sunday.

Tear gas was initially deployed but failed to disperse the group.

Later, a water cannon against a makeshift barricade – that marked the first time water cannon vans had ever been used in the city. Protesters were pushed back, but again the crowd didn't disperse.

As the violence escalated, six officers drew their pistols after being surrounded by armed protesters and one officer fired a live shot into the air, police said.

The Hong Kong police said in a statement that the officers were “surrounded, under attacks and facing threats to life” and one officer fired “a warning shot to the sky without any other choices.”

A 12-year-old was among the 36 people arrested on Sunday, underscoring that young children are involved in the more violent aspects of the protest movement, days before schools reopen after the summer break. Others arrested include 29 men and seven women between the ages of 12 and 48, for offences including unlawful assembly, possession of an offensive weapon and assaulting police officers, police said.

Week of calm shattered

Violence also broke out on Saturday, after thousands in the city's east Kwun Tong district marched for the movement's five demands, and against the government's installation of “smart” environmental monitoring lampposts, which have sparked privacy concerns.

The standoff brought an end to a week of relative calm in Hong Kong, which had seen numerous clashes between protesters and police since early summer. After tear gas was fired nearly every weekend in July, last weekend saw an estimated 1.7 million people brave the heavy rain and heat to march peacefully. The calm continued through the week, with protesters peacefully creating a human chain across the city on Friday – the 30th anniversary of the Baltic Way human chain.

However, senior police officials, who spoke on condition of anonymity, said this week that officers had been targeted and exposed online even while there was temporary peace on the streets. The police said officers’ personal data, contact information, home addresses and more had been shared online, and accused protesters of threatening officers’ families.

Families of police speak out

A group of Hong Kong Police relatives held their own rally on Sunday, calling for a communication platform between the police and the public “to mend the broken relationship,” and asking the police to act “with malice or ill-will toward none,” according to a Facebook post by organizers.

The group of about 400 attempted to deliver a letter to Chief Executive Carrie Lam, saying that their relatives in the police force should not be “used as a shield” and that Lam should “settle the political issue with politics.”

The group did not denigrate the protest movement, instead said that the police force were “being pushed by political issues to confront the public.”

The nonstop protests have taken a toll on everyone involved – on Saturday, Lam had posted a lengthy statement on Facebook appealing for peace and dialogue.

“After more than two months, everyone is tired. Can we sit down and talk about it?” she said, acknowledging that there were deeper societal problems beyond the immediate violence that needed to be addressed.