As of 8 pm local time (1200 GMT), eight hours after online polling opened, 298,268 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, which will be held over 10 days, said a high turnout would prove the city's determination to win "true" universal suffrage.
Under the "one country, two systems" agreement reached when Hong Kong was handed over from former colonial power Britain to Communist-ruled China in 1997, Hong Kong has guaranteed civil liberties not enjoyed on the mainland, including free speech and the right to protest.
The semi-autonomous 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.
"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.
Polling will continue until June 29 and residents have so far cast their votes online and via smartphones. There are plans 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.
"All three options on the ballot are against the Basic Law," Leung said, referring to Hong Kong's constitution.
