Online polling started yesterday at noon and 500,436 residents had taken part in the "civil referendum", which asks how voters would like to choose their next leader, by 3:00 pm local time today.
The former British colony's leader is currently appointed by a 1,200-strong pro-Beijing committee but there have been increasingly vocal calls for residents to be able to choose who can run for the chief executive post.
China has promised direct elections for the next chief executive in 2017, but has ruled out allowing voters to choose which candidates can stand.
The pro-democracy group said before launching the exercise, which runs until June 29, that they were hoping for 300,000 people to take part.
Also Read
The 500,000 who had voted in the first 27 hours of the poll represents a sizeable chunk of the 3.47 million people who registered to vote at elections in 2012.
The ballot allows registered permanent residents of the semi-autonomous city to vote through a website or on a smartphone app and there are plans to open polling booths around the city tomorrow.
"We have reasonable suspicion to believe that Beijing was behind the attacks because which authority would have the resources and motivation?" Benny Tai, one of the founders of the Occupy Central movement, told AFP.
"It's unprecedented in Hong Kong," Tai said of the size of the attack.
Under the "one country, two systems" agreement reached when the city of seven million people was handed over from 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.
Many pro-democracy activists fear that Beijing will handpick the candidates to ensure that the job does not go to anyone who would be critical of China.