By Emily Chung and Nikki Sun
HONG KONG, June 20 (Reuters) - Tens of thousands of people voted for full democracy in Hong Kong within the first hour of an unofficial online referendum, a civil campaign that has sparked warnings from China's Communist Party leaders.
Social tensions have steadily risen in the former British colony that returned to Chinese rule in 1997 - with pro-democracy activists threatening to blockade part of the city's financial district if China doesn't allow opposition candidates to run in a 2017 election.
While Beijing says Hong Kong can go ahead with a city-wide vote in 2017 for the city's top leader, 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.
Instead, Beijing insists a small committee of largely pro-Beijing loyalists choose who gets on the ballot, which would effectively render the ability to vote meaningless.
Even with the 'PopeVote' website functioning only intermittently after a cyber-attack earlier in the week, more than 50,000 votes were cast within an hour and a half of opening, said pro-democracy activists who organised the ballot. Most of the votes were cast through a smartphone app.
Some 37,000 pre-registered to vote in the days beforehand.
A spokesman for the Hong Kong government said internet vote had "no legal effect" and there was no provision under Hong Kong laws for such a vote.
Around 200 supporters marched peacefully in the rain to Hong Kong government headquarters, singing, waving banners and urging more people to participate in the symbolic call for full democracy at a critical juncture for the city.
Government and police sources have warned of the risk of other radical groups joining or 'hijacking' the so-called Occupy Central movement, with police on Friday bolstering security around the legislature after a small band of demonstrators tried ramming their way into the building last week.
Pro-Beijing newspapers, Chinese officials and Hong Kong business tycoons have strongly criticised the Occupy Central campaign, which plans mass protests in the Central business district this summer, saying it will harm Hong Kong.
Activists say it is a 'peaceful movement' demanding a 'genuine choice' for Hong Kong's 5 million eligible voters.
One of the organisers of the online vote, academic Benny Tai, stressed his movement hadn't yet decided on an exact date to launch the street protests, though the results of the online vote would have a strong bearing.
Banks in Central have been holding emergency drills and contingency planning for possible disruptions to operations.
Hong Kong returned to China with wide-ranging autonomy under the formula of "one country, two systems" - along with an undated promise of full democracy, an issue never broached by the British until the dying days of 150 years of colonial rule.
The referendum is seen as an important test for pro-democracy activists who are increasingly embittered by perceptions of China's increased control over the city that was promised broad-ranging freedoms upon its return to Chinese rule.
Last week, Beijing released a white paper reasserting Beijing's total control over Hong Kong in what many saw as a veiled warning.
The online vote will be open until June 29. (Additional reporting by Adam Rose, Clare Baldwin and James Zhang; Writing by James Pomfret; Editing by Jeremy Laurence)