Hong Kong holds a controversial "referendum" on democracy on Friday, a prelude to an escalating campaign of dissent that could shut down the former British colony's financial district and further anger China's Communist Party leaders.

An affluent city of 7 million that returned to Chinese rule in 1997, Hong Kong's long-standing push for full democracy is reaching what could be the boiling point with tens of thousands expected to vote in the unofficial referendum for full democracy from Friday until Sunday.

While Beijing has allowed Hong Kong to go ahead with a popular vote for the city's top leader in 2017, the most far-reaching experiment in democracy in China since the Communist takeover in 1949, senior Chinese officials have ruled out allowing the public to nominate candidates.