As of 4 pm local time, four hours after online polling opened, 166,980 people had taken part in the informal "civil referendum" which asks participants to choose between three methods to vote for Hong Kong's next leader in 2017.
Occupy Central, the local pro-democracy movement that organised the vote, said a high turnout would prove the city's determination to win "true" universal suffrage for the semi-autonomous city.
The city's leader, or chief executive, is currently appointed by a pro-Beijing committee.
China has promised direct elections for the next chief executive in 2017 - but has ruled out allowing voters to choose which candidates can stand.
Many pro-democrats fear Beijing will hand-pick the candidates to ensure election of a sympathetic official.
"If we could get a good turnout, that shows that Hong Kong people are really determined to have true democracy," Benny Tai, one of Occupy Central's founders, said at a launch event for today's vote.
Residents have so far cast their votes online and via smartphones, but organisers are planning to open polling booths around the city on Sunday.
Chinese authorities said today that any referendum in Hong Kong on how to elect its leader would not have constitutional grounds and would be illegal and invalid, state news agency Xinhua reported.
Occupy Central is planning to paralyse Hong Kong's financial district with thousands of protesters at the end of the year, as a last resort if officials do not allow voters to choose their own candidates.
