Nearly 600,000 votes have been cast in three days of an unofficial referendum on democratic reforms in Hong Kong, part of a civil campaign that has been branded illegal by the former British colony and by Communist Party authorities in Beijing.

Hong Kong returned to Chinese rule in 1997 with wide-ranging autonomy under the formula of "one country, two systems", along with an undated promise of universal suffrage.

But social tensions have risen steadily, with many residents concerned that civil liberties are being eroded and with pro-democracy activists threatening to blockade part of the city's financial district if China doesn't stick to its promise.

Polling in the unofficial referendum was extended from June 22 until June 29 after the voting website received billions of hits in an apparent cyberattack last week that prompted organisers to open 15 polling booths across the city on Sunday. Residents have so far voted online and via smartphones.

While Beijing says Hong Kong can go ahead with a vote in 2017 for the city's top leader, Hong Kong's mini-constitution, the Basic Law, specifies that only a nominating committee can pick leadership candidates. Pro-democracy activists say this should be changed to allow public nominations.

"What is the point of one man, one vote if at the end of the day we have to vote from three puppets or four puppets anointed by Beijing?" Anson Chan, Hong Kong's former top civil servant and a key supporter of the vote, said on Sunday.

The unofficial referendum comes a week after Beijing released a white paper in which it reasserted its authority over Hong Kong, a move that unnerved and enraged many in the city, raising fears of direct intervention.

At one polling station, about 20 people from pro-Beijing group Caring Hong Kong Power shouted at voters who braved the rain, urging them not to cast ballots.

"You are sending your children to the battlefield," one shouted at a voter.

The referendum, organized by the Occupy Central protest group, offers voters three options, which all include a popular vote. There is also an option to abstain, but no breakdown of the results has been provided.

More than 590,000 votes had been cast by midday on Sunday, according to organisers, or nearly 10 per cent of Hong Kong's population of seven million.

Occupy Central had said it hoped the referendum would draw up to 300,000 people and it aims to mobilize thousands on to the streets of the city's financial district as part of its campaign to demand full democracy.

It's been a busy few weeks for pro- and anti-establishment groups. Pro-Beijing group Silent Majority released a video last week warning of possible deaths and general mayhem if the Occupy Central protests proceed. In one scene, a dagger pierces a throbbing heart that symbolizes the central business district.

At a sing-along rally in support of the referendum on Friday, more than 100 people gathered in Central to sing "Do You Hear the People Sing?" from the musical Les Miserables.

Pro-Beijing newspapers, Chinese officials and Hong Kong business tycoons have strongly criticized the Occupy Central campaign, saying it will harm Hong Kong.

Activists say it is a peaceful movement demanding a "genuine choice" for Hong Kong voters.-Reuters