What is China's People's Liberation Army doing in Hong Kong?

Hong Kong (CNN) -- Just yards from where thousands of pro-democracy supporters have been "occupying" the heart of Hong Kong's financial district, stands an imposing gray building surrounded on all sides by a high wall -- this is the Hong Kong garrison of the People's Liberation Army (PLA), China's armed forces.

The main gates of the base, known as the Prince of Wales barracks until London handed over the city to Beijing in 1997, are guarded by heavily armed sentries in green combat fatigues, who stand statue-like with impassive expressions 24 hours a day, seven days a week.

But despite the fact Hong Kong is a Chinese territory, the PLA garrison abides by local Hong Kong laws that emphasize the city's considerable autonomy. In short they keep a low profile: the soldiers never come out onto the streets of the city, and there's minimal interaction with the local population beyond the occasional open day.

While there is nothing to suggest this will happen, the possible intervention of the 6,000 PLA troops believed stationed here has been the "elephant in the room" for many, as we approach the first full week of pro-democracy demonstrations. For some protesters, memories are still fresh of 1989 and the brutal crackdown of student protests in Beijing's Tiananmen Square.

PLA arrives

It was July 1, 1997 when the main force of Chinese soldiers rolled across the border from the southern Chinese city of Shenzhen into Hong Kong, as Britain formally ceded control of the territory it had controlled since 1841.

Despite heavy rain, the convoy of green military trucks filled with soldiers clutching their rifles was met by crowds waving the flag of the nascent Special Administrative Region of Hong Kong: a white five-petal bauhinia flower on a red background. They headed for Hong Kong Island and their new base, which had earlier been vacated by a British military detachment and was now flying the flag of the People's Republic of China.