More than 780,000 votes were cast by Sunday, the final day of an unofficial referendum on how Hong Kong's next leader should be chosen.
The ballot has been branded illegal by local and mainland Chinese authorities.
Hong Kong, a free-wheeling, capitalist hub of more than 7 million people, returned to Chinese rule on July 1, 1997, with wide-ranging autonomy under a “one country, two systems” formula, along with an undated promise of universal suffrage.
China has promised to let all Hong Kong residents vote for their next leader in 2017. But it said candidates must be approved by a nomination committee.
Pro-democracy advocates are incensed at current plans for the election of Hong Kong's next chief executive - who is currently appointed by a 1,200-strong pro-Beijing committee.
Annual protest rally
Tensions are running high in the former British colony before the anniversary of its handover to China, a traditional day of protest.
Organizers of Tuesday's rally expect it to be the largest since the handover with upwards of 500,000 people expected, as frustration grows over Beijing's tightening control over the city.
"Public sentiment has dropped to the lowest point since 2003. I believe more people will come out," Johnson Yeung, one of the organizers, told AFP.
Democracy activists want the nomination process to be open to everyone, in line with international standards, and have threatened to lock down the Central area of Hong Kong, home to some of Asia's biggest companies and banks, if the city fails to adopt a strong democratic method for electing its next leader.
“I think the signal has already been sent to Beijing that Hong Kong people are prepared to express their views on universal suffrage,” said Benny Tai, associate professor of law at the University of Hong Kong and one of the organizers of the vote and the movement, Occupy Central with Love and Peace.
“We hope the result of the civil referendum will be taken seriously by the SAR (Special Administrative Region of Hong Kong) and Chinese government.”
Supporters of Caring Hong Kong Power, a pro-China group, march to the police headquarters during a demonstration against an unofficial referendum and the so-called Occupy Central protest movement in Hong Kong, June 29, 2014.
The unofficial 10-day vote, organized by pro-democracy activists, was conducted partly online and partly at physical ballot boxes. Voters were given three options on how the next chief executive should be chosen.
Each would allow voters to propose candidates for the top job, and all are therefore considered unacceptable by China and the Hong Kong government.
Voters are required to give their identification number to prevent cheating.
At a “polling booth” at Chinese University of Hong Kong on Sunday, a small group of pro-Beijing supporters with mainland accents held up banners denouncing the vote, while four people jumped into the city's Victoria Harbor to protest against the referendum and were quickly rescued.
Another pro-Beijing group, Caring Hong Kong Power, marched through the busy shopping district of Causeway Bay carrying bright orange balloons and urging people not to vote.
Group spokeswoman Lee Ka-ka handed a petition to police signed by 30,000 against the Occupy Central group. She also urged police to “act strongly against the movement.”
Results of the online referendum are expected to be released at around 11 p.m. local time on Sunday, with the overall tally set to be announced on Monday.
Visitors take aim with rifles at a military base during an open day event of the Chinese People's Liberation Army (PLA) in Hong Kong, June 29, 2014.
The last day of voting coincided with China's military opening its barracks in Hong Kong to the public, giving curious tourists a rare glimpse inside two outposts, as tensions between local democracy activists and Beijing continues to heat up.
The 10-day poll, organized by Occupy Central, comes at a time when many Hong Kong residents fear civil liberties are being eroded and amid growing concern about the rule of law in the Asian financial center.
Civil liberty fears
On Friday, Hong Kong lawyers dressed in black marched through the city to protest against the wording in a white paper released this month by Beijing in which it said being patriotic and “loving the country” is a basic requirement for the city's administrators, including lawyers.
The lawyers were taunted by pro-Beijing groups shouting into loud hailers as they marched to the Court of Final Appeal.
Many recent rallies in Hong Kong have seen scuffles break out between pro and anti-Beijing groups, including the 25th anniversary of the June 4, 1989, crackdown on pro-democracy protests in and around Beijing's Tiananmen Square, an event that had always been peaceful in Hong Kong.
Tai said on Sunday the white paper, which reasserted Beijing's control over the former British colony, had “backfired” and prompted more people to vote.
Pro-Beijing newspapers, Chinese officials and Hong Kong business tycoons have strongly criticized the Occupy Central campaign, saying it could hurt the city's standing as a financial center.
The big four audit firms were the latest to join the chorus, when they took out adverts in local Hong Kong newspapers on Friday warning that investors could leave the city if mass protests go ahead.
Activists say it is a peaceful movement demanding a “genuine choice” for Hong Kong's voters.
The unofficial referendum is seen as an important test for pro-democracy activists who believe the public are dissatisfied with the pace of political reform promised by Beijing.
Some information for this report provided by Reuters and AFP.