HONG KONG (Reuters) - Among dozens of suspected pro-Beijing groups registered with Hong Kong authorities as companies or societies, one is conspicuous by its absence - the Communist Party itself.
While Beijing is playing an increasing role in Hong Kong affairs, the local branch of the ruling Communist Party remains officially underground in the free-wheeling capitalist hub - a legacy, in part, of British colonial rule that ended in 1997.
From helping foment a wave of riots in Hong Kong in the 1960s that challenged British rule, to paving the way for China's reunification, the Communist Party is deeply embedded in the city's history.
"Nowhere else in the world is there a system where the ruling party remains an underground organisation as it does in Hong Kong," wrote Christine Loh in her book "Underground Front: The Chinese Communist Party in Hong Kong."
"Its presence in Hong Kong is meant to be a 'secret' although one that everyone has known about for a very long time," added Loh, who is now a senior Hong Kong official.
Nobody really knows how many Communist Party members there might be in Hong Kong, or how powerful it is.
Hong Kong's current leader, Chief Executive Leung Chun-ying, has had to repeatedly deny rumours he is a Communist Party member.
"I am not a member of the Communist Party. I am not a so-called underground member of the Communist Party. In fact, I'm not a member, and have not been a member, of any political party anywhere in the world," he told Reuters in 2012.
The British never actually outlawed the party. Sonny Lo, author of a definitive book on China's underground control of Hong Kong, notes that London reached a "tacit understanding" after the Communist takeover of China in 1949 that Hong Kong could not be used as a battlefront between communists and the defeated Kuomintang nationalists that fled to Taiwan. Both would therefore have to remain underground.
The Party, which traces its origins in the city back to 1920s, operated through a secret committee, running schools, unions and newspapers. China's state Xinhua news agency office became its unofficial headquarters in Hong Kong.
To ensure a successful handover - and its promise of "one country, two systems" - Beijing opted for the status quo and no explicit role for the Party after Hong Kong returned to Chinese rule in 1997.
Xinhua was reduced in size -- its old building has largely been turned into a state-owned hotel -- and Party work moved to the new Liaison Office in 1998, a year after the handover.
No mention is made of the Party in the Basic Law, Hong Kong's mini constitution that outlines its freedoms and autonomy and governs its relationship with the rest of China.
Nor is there any significant public clamour for change - even though the situation is at the heart of a clash of political cultures as Beijing's confronts a Hong Kong populace eager for full democracy.
Across university campuses in Hong Kong, the mainland Chinese Students and Scholars Association has been active in organising underground party meetings, according to sources.
"I've got mainland students who have flat out told me they are party members, and even that they go to (the) Liaison Office for meetings," one professor said, speaking on condition of anonymity.
"Things can only get more complicated here as the communist shadows grow longer. That is Hong Kong's reality."
(Editing by Bill Tarrant)