Hong Kong police said on Monday they arrested 36 people, the youngest aged 12, after violence during anti-government demonstrations escalated as protesters hurled Molotov cocktails at security forces who responded with water cannon and tear gas.

Sunday’s protests saw some of the fiercest clashes yet between police and demonstrators since violence escalated in mid-June over a now-suspended extradition bill that would have allowed Hong Kong people to be sent to mainland China for trial.

Police fired water cannon and volleys of tear gas in running battles with brick-throwing protesters on Sunday, the second day of violent clashes in the Chinese-ruled city.

Six officers drew their pistols and one officer fired a warning shot into the air, police said in a statement.

“The escalating illegal and violent acts of radical protesters are not only outrageous, they also push Hong Kong to the verge of a very dangerous situation,” the government said in a statement.

Protesters once again adopted cat-and-mouse tactics late in the evening, moving swiftly to locations across the former British colony, where they set up barricades to block some roads, following a largely peaceful rally earlier in the day.

Police said they arrested 29 men and seven women, aged 12 to 48, for offenses including unlawful assembly, possession of offensive weapons and assaulting police officers.

The clashes on Saturday and Sunday marked a return to unrest after days of calmer demonstrations. The protests, which escalated in June over a now-suspended extradition bill, have rocked Hong Kong for three months, occasionally causing serious disruption including forcing the closure of the airport.

The city, a major Asian financial center, is facing its biggest political crisis since the handover of power from British rule in 1997.

Protesters say they are fighting the erosion of the “one country, two systems” arrangement under which Hong Kong returned to China with the promise of continued freedoms, not enjoyed on the mainland, for 50 years.

But in a sign that Beijing may be losing patience, China sent the strongest warning yet it’s thinking of using troops on Hong Kong’s streets.

“It’s not only China central government’s authority but also its responsibility to intervene when riots take place in Hong Kong,” the state-run Xinhua News Agency said Sunday in a commentary, drawing on comments by former top leader Deng Xiaoping saying Beijing has to act under such circumstances.

U.S. President Donald Trump said Aug. 13 that reports from the country’s intelligence agencies show the Chinese government is moving troops to its border with Hong Kong. A day earlier, Global Times, a Chinese tabloid run by the People’s Daily, reported that the Chinese People’s Armed Police were assembling in Shenzhen ahead of “apparent large-scale exercises,” where “numerous” armored personnel carriers, trucks and other vehicles of the paramilitary force were seen heading toward Hong Kong’s neighboring city.

In Sunday’s commentary, Xinhua said Hong Kong’s protests have turned into a “Color Revolution” aimed at overturning the Special Administrative Region’s constitutional institutions, a signal it was ready to take further action. Previously, Chinese officials had described the protests as having some characteristics of a “color revolution.”

The Hong Kong police defended its officer’s decision on Sunday night to fire a shot in the sky, calling it “the best option” to disperse hundreds of protesters who were charging toward a fallen officer with metal poles and other weapons. Six officers had their guns drawn.

“Our officer’s life was in great danger,” Yolanda Yu, a police senior superintendent, said at a briefing early Monday morning, which started with video footage of black-clad protesters attacking the police. “The use of force was indeed necessary and reasonable — it was to protect any person, including the officer himself, from death or serious bodily injury.”

About 15 officers were hurt in the clashes as Yu also stood by the move by an officer, who while holding his revolver, had kicked a protester. She called it a “natural reflex.”