A university website offering ordinary Hong Kongers a chance to vote for their next leader ahead of Sunday's election is under "systematic attack" from hackers, organisers said Friday.

Thousands of people who do not have the right to vote in the tightly controlled "small-circle election" are expressing their views through the unofficial poll organised by the University of Hong Kong.

"The system has been very busy," Robert Chung, director of the university's respected Public Opinion Programme, told Cable TV.

"We suspect it is under systematic attack as there are over one million clicks on our system every second."

Chung did not indicate who could be responsible for the disruption, but his team of pollsters has a history of aggravating mainland authorities with surveys indicating public opinion that is at odds with Beijing's official line.

The vast majority of Hong Kong's seven million residents have no right to determine who will replace Chief Executive Donald Tsang, whose term expires in June, as the southern financial centre's next leader.

The vote is restricted to a 1,200-member committee packed with pro-Beijing elites, who will cast their ballots on Sunday.

The leading candidates are Henry Tang, the heir to a textile fortune and former chief secretary, and Leung Chun-ying, a policeman's son turned self-made property consultant -- both regarded as pro-Beijing establishment figures.

The university's so-called civil referendum began at midnight Thursday and continues until 9:00pm (1300 GMT) Friday, with ballots cast through the website, via mobile phones or at 15 "polling stations".

"This is a meaningful exercise because it shows Hong Kong people want direct elections," university lecturer Michele Ho told AFP after casting her vote at a booth.

"Although we can't influence the election outcome on Sunday, this mock vote shows we want to have a say and we should have the right to decide who is our next leader."

Before the polls opened organisers said they had hoped to attract at least five million votes.

Hong Kongers were also expressing themselves through a free iPhone app called "I want to vote for my chief executive".

The version for iPhones had more than 4,200 votes by 1:30pm on Friday, with Leung leading on 34.6 percent.

He was closely followed by blank or donkey votes on 32 percent. Tang was next on 19.2 percent and third candidate Albert Ho had 14.9 percent of the vote.

Tang was seen as China's choice until a slew of scandals and gaffes made him unpopular with the public. Even so, he continues to enjoy the strong support of key committee members including Asia's richest man, Li Ka-shing.

Hong Kong reverted to Chinese control from British rule in 1997, with a semi-autonomous status that guarantees broad social freedoms under limited democracy.

Beijing has said that, at the earliest, the city's chief executive could be directly elected in 2017 and the legislature by 2020.