Organizers said 230,000 people cast online ballots in the first hour of the 10-day survey in favor of direct election of the city's chief executive, an indication voters seek less involvement by Beijing in the city's affairs.
Hong Kong, a British colony until 1997 and a city of seven million, currently has its leadership chosen by a pro-Beijing committee.
Voting began Friday and will continue until June 29.
China will permit direct election of the Hong Kong chief executive beginning in 2017, but pro-democracy activists believe he or she will come only from a list of candidates approved by Beijing.
The referendum has no official standing, and a website and smartphone app -- the two methods of voting -- were overwhelmed with billions of cyber-attacks after they opened last week.
Chinese officials and pro-Beijing media have been critical of the vote, suspecting it could lead to growing dissent in Hong Kong. Threats to shut down Hong Kong's business district have been made by pro-democracy factions if opposition candidates cannot run for the chief executive position in 2017.