By Greg Torode and David Lague
HONG KONG (Reuters) - Major General Tan Benhong, the commander of the People's Liberation Army in Hong Kong, was a picture of uniformed calm as he shared champagne toasts with Chinese officials on Wednesday at local celebrations marking China's national day.
The streets surrounding the bash at the Hong Kong Convention Centre presented a starkly different scene as thousands of protesters escalated the most sustained push for full democracy since China took Hong Kong back from Britain in 1997.
As the protests enter their second week amid fresh signs of street violence, some demonstrators and ordinary Hong Kong citizens fear Tan's troops could eventually be ordered to crush a movement unthinkable on the mainland.
Thorny political, legal and strategic realities make any such involvement of the PLA exceptionally difficult, however, and Hong Kong's 27,000-strong police force is expected to remain in charge for the time being.
Government advisers and experts believe leaders in both Beijing and Hong Kong understand the immense political costs of ordering the PLA out of their barracks, ending at a stroke Hong Kong's vaunted autonomy under the "one country-two systems" formula under which Britain agreed to hand over the Asian financial hub.
Foreign diplomats are monitoring developments closely, noting moves in recent months to upgrade PLA facilities in Hong Kong and unconfirmed reports of anti-riot drills being staged at both urban and rural bases.
The garrison comprises between 8,000 to 10,000 personnel, mostly infantry troops, spread between bases across the border in Shenzhen and in Hong Kong, envoys estimate. It includes a small naval and air-force attachment.
"I think that (Hong Kong) policymakers at the highest level ... are fully aware in that if the PLA were deployed, in the eyes of the world it would be the end of one country-two systems," Regina Ip, an adviser to embattled Hong Kong leader Leung Chun-ying and a former security chief, told Reuters.
"It would cause tremendous damage," she said, noting that the local government officials had repeatedly stressed that local police - equipped with paramilitary anti-riot units - were fully capable of dealing with the unfolding situation.
A Hong Kong-based mainland security academic said he believed Beijing was also acutely aware of the risks of using the PLA in Hong Kong.
"I am sure the leadership in Beijing knows that any such involvement would involve massive political costs," he said, speaking on condition of anonymity given the sensitivity of discussing PLA operations.
"For that reason, the PLA in Hong Kong is maintained largely as a symbolic presence."
RISE OF RED NEON STAR
Foreign diplomats watching developments remain unsure of the precise political threshold that must be crossed for Beijing to unleash the troops.
Their presence in Hong Kong, after all, has been one of the most sensitive elements of the handover, fuelled by deep local memories of the 1989 army crackdown on protesting students in and around Tiananmen Square in Beijing - an event still commemorated annually in the city.
The Chinese military is housed in 19 urban and rural sites inherited after the British folded up the Union Jack on what had been a major military staging post of the British empire and a vital Cold War-era listening post.The bases, dotted strategically across Hong Kong island, Kowloon and the New Territories, include the old British Tamar headquarters building in Admiralty, now next door to the government's new offices - and the epicentre of the current demonstrations.
The building was recently refurbished, along with other sites, and is now topped with a red neon star. Most of its offices are thought to be occupied, some for the first time.
A large domed surveillance camera sits perched above a corner over one of the main thoroughfares leading to Hong Kong's glittering Central financial district.
Both Asian and Western envoys say they have spotted a lot more activity both at the headquarters and other sites over the last year. Unmarked black SUV vehicles sporting military number plates are now a common sight on city streets.
"We are just not yet sure whether the activity is related to the Hong Kong situation, or reflects China's broader military build-up," one Asian envoy said.
The garrison did not respond to questions from Reuters.
MAINLAND SOLDIERS KEEP TO THEMSELVES
The PLA's presence is controlled by Hong Kong's mini-constitution, the Basic Law, which states the garrison must not "interfere" in the city's affairs but that Hong Kong's leaders can request its help to keep order or handle disasters.
They must abide by local laws, which are governed by an independent judiciary, in contrast to the mainland legal system.
Other parts of the Basic Law echo mainland laws and appear to allow for the standing committee of China's parliament, the National People's Congress (NPC), to deploy the garrison if a state of war or emergency is declared for Hong Kong.
Under the Basic Law, the NPC would need to be satisfied that there was enough "turmoil" to endanger "national unity or security" and was "beyond control of the (Hong Kong) government".
"Ultimately I think it would have to be pretty extreme to invoke those provisions and call in the garrison," said Hong Kong University law professor Simon Young. "We're really talking complete collapse of government control or civil war."
One foreign envoy said the laws were written with enough leeway for Beijing to "create a narrative under which deployment becomes imperative".
The mainland law covering the garrison does not allow soldiers in Hong Kong to belong to local political, social or religious organisations. None are recruited locally. They are kept on base where they are effectively isolated from Hong Kong's freewheeling society and media.
British troops enjoyed far more freedoms, and filled the city's bar districts on nights off. Other than occasional open-days, there is very little interaction between the PLA and the Hong Kong community.
And unlike the British forces in Hong Kong, the PLA command is kept at arms length from the local government, liaising via its civilian-run security bureau.
There is no joint police-army operational wing as in the colonial era, when the chief of British forces also served on the governor's executive council, effectively his cabinet.
Foreign envoys note that the garrison appears largely geared for internal security, rather than repelling foreign invasions.
Relatively lightly-armed, the garrison's chief weapons are lightly armoured WZ523 personnel carriers, equipped with machine guns and suitable for Hong Kong's tight and congested streets.
Internal security was stressed, too, when Major-General Tan led the toasts during cocktails with foreign diplomats, local officials and police as well as mainland government officials and businessmen to mark the 87th anniversary of the PLA's founding on July 31.
It was something of a coming out party for Tan, who had just arrived in Hong Kong after commanding military forces on the strategically important Hainan Island in the South China Sea.
The 57-year-old has also served in elite Second Artillery command that controls China's conventional and nuclear missiles, and earlier did a spell as garrison chief-of-staff in Hong Kong.
He has said he would command jointly with his political commissar, the more senior Lieutenant General Yue Shixin, a signal of the importance Beijing places on the political dimension of the garrison's Hong Kong role.
The PLA, he told the gathering, had a long tradition of ensuring China's internal stability and security.
"It was a very short, blunt speech," said one observer. "Then there was all this Russian-style martial music blaring away. It was hardly the usual Hong Kong cocktail party."
(Editing by Mark Bendeich and Jeremy Laurence)