Hong Kong returned to Chinese rule in 1997 with wide-ranging autonomy under the formula of "one country, two systems", along with an undated promise of universal suffrage.

But social tensions have risen steadily, with many residents concerned that civil liberties are being eroded and with pro-democracy activists threatening to blockade part of the city's financial district if China doesn't stick to its promise.

Polling in the unofficical referendum was extended from June 22 until June 29 after the voting website received billions of hits in an apparent cyberattack last week that prompted organisers to open 15 polling booths across the city on Sunday. Residents have so far voted online and via smartphones.

While Beijing says Hong Kong can go ahead with a vote in 2017 for the city's top leader, Hong Kong's mini-constitution, the Basic Law, specifies that only a nominating committee can pick leadership candidates. Pro-democracy activists say this should be changed to allow public nominations.