Organisers of the Occupy Central with Love and Peace movement said that in the first six hours about 165,000 ballots were cast on proposals for electoral reform.
They hope at least 300,000 people will take part.
Tensions are boiling over in the former British territory, which came back under Chinese control in 1997, over how to choose the city's next leader.
Since the end of British rule, Hong Kong's leaders have been picked by an elite pro-Chinese committee.
Beijing has pledged to allow the people of Hong Kong to choose their own chief executive starting in 2017.
All three options on yesterday's ballot called for the public to nominate candidates, which Beijing has rejected, insisting candidates be vetted by a Beijing-friendly committee.
Most ballots were cast by phone with tens of thousands more cast online despite disruption from a cyberattack.