Men in riot helmets and military fatigues also shone a spotlight down on the street where some protesters were shining laser pens at the building as they walked past. One of the men reportedly shouted at the protesters in Cantonese, which is also spoken in the closest mainland city of Shenzhen. An 18-year-old student and a 38-year-old unemployed woman on Monday became the first two protesters to be prosecuted for wearing masks. Also charged with taking part in unlawful assembly, the pair, who were detained on Saturday soon after the new laws came into effect, face jail terms of up to six years. Until this weekend, the PLA garrison has stayed out of sight as clashes between protesters and Hong Kong police have flared on the streets, including previous instances where Molotov cocktails have been thrown within metres of another PLA barracks on Hong Kong island. So the appearance of the yellow warning flag from the barracks has renewed fears that Beijing may lose patience after 18 weeks of protests.

Riot police use pepper spray against protesters. Credit:Bloomberg Hong Kong's Basic Law allows the the PLA to be deployed to restore public order if a request is made by the Hong Kong government. On Friday, Hong Kong Chief Executive Carrie Lam declared the city was not in a state of emergency, but repeatedly made the case Hong Kong was in the grip of "serious public danger" to justify using emergency powers for the first time in 50 years to restore order. The Kowloon Tong barracks is one of 19 Chinese military sites in Hong Kong and the New Territories controlled by the PLA garrison. The Chinese troops rotate annually but on August 29, the last rotation, no mention was made of troops leaving, only new troops arriving, suggesting the number of troops had increased.

The protests in Hong Kong became heated over the weekend. Credit:Getty Images Reuters reported last month that foreign envoys estimated there were now 10,000 PLA troops in Hong Kong. The Hong Kong police force has 30,000 officers and Lam had expressed concern that police were outnumbered by the public in a leaked audio recording last month. Protesters flooded the streets on Friday after the announcement of the emergency laws. Credit:Getty Images Large marches on Sunday in Kowloon and Hong Kong island protesting against the emergency law were dispersed with tear gas.

A government statement issued on Sunday night urged the public not to behave illegally. Loading Members of "the public" had participated in unauthorised assemblies, where "a large number of masked rioters took the opportunities to block roads and vandalise public property, MTR stations as well as banks and shops selected by the rioters", the government statement said. Petrol bombs had been thrown at police and stores seen as pro-Beijing were ransacked. "Violent acts in recent days have thrown the community into panic and chaos, affecting many public services and prompting malls and shops to suspend businesses for fear of vandalism," the statement said.

There was an "extensive and imminent danger to society". But Jason Ng, the convener of the Progressive Lawyers Group, asked: "has common sense been switched off?" after video footage of children being arrested on Sunday during the large march was circulated. In one video, a girl is visibly shaking as she is surrounded by riot police twice her size. Protesters are arrested during clashes. Credit:Getty Images Large sections of the train network were again shut down on Sunday night, while buses were stopped and passengers searched by police for masks at multiple locations.

Hong Kong Baptist University said five students were arrested after police entered the university without permission or a warrant on Sunday. Loading In a statement at midnight, the government denied rumours that Mrs Lam would use emergency powers to suspend the stockmarket and schools when the holiday weekend finished on Tuesday. Rumours that daily cash withdrawals from banks would be capped were also denied by the Hong Kong Monetary Authority. "The Hong Kong banking system is robust and sound and banks have sufficient supply of banknotes to meet the needs of the public," it said.