It's business as usual for US military's social media sites despite the fact that hackers broke into the Pentagon's Twitter account.

Hong Kong - A university website offering Hong Kongers a chance to vote for their next leader ahead of Sunday's official election is under “systematic attack” from unidentified hackers, organisers said on 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.

But organiser Robert Chung, director of the university's respected Public Opinion Programme, said the site had been swamped by denial-of-service type attacks designed to saturate and overload the server.

Some of his colleagues' passwords also had been mysteriously changed in recent days, he added.

“We found incidents of abnormally high hit rates on March 21 ... We registered about a million hits per second. We think there could not be another reason other than cyber attacks on us,” he told reporters.

“This morning, shortly after 7:00am, we had another attack which basically tore down our servers, so we couldn't conduct any electronic voting.

“The abnormal number of hits have exceeded what we can predict or control.”

He said the disruptions were “indications of high level attacks” but he did not speculate about who could be responsible.

Chung's team of pollsters has a history of aggravating mainland authorities with authoritative surveys indicating public opinion that is at odds with Beijing's official line.

“What we wish to do now is try our best to satisfy the public's expectations, which is to provide a platform for them to express their opinions,” he said.

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 Chinese city's next leader.

The vote is restricted to a 1,200-member committee packed with pro-Beijing elites including powerful business tycoons with close ties to the mainland.

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

Heir of a textile fortune Henry Tang was considered a shoo-in for the leadership thanks to his ties to industrialists and bankers, who have run the city in partnership with Beijing since the handover.

But a slew of scandals made him deeply unpopular with the public and embarrassed his backers in Beijing, who reportedly shifted their support at the last minute to his rival Leung Chun-ying.

A policeman's son turned self-made property consultant, Leung has consistently outscored Tang in approval rankings but would still struggle to win a majority of votes in a genuinely democratic system.

Both men are regarded as pro-Beijing establishment figures with few ideas for addressing the issues that concern ordinary Hong Kongers, such as the yawning gap between rich and poor and sky-high property prices.

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

“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,” academic Michele Ho told AFP after casting her vote at a booth.

Hong Kongers were also having their say through a free iPhone app called “I want to vote for my chief executive”.

The app had more than 4,200 votes by 1:30pm on Friday, with Leung leading on 34.6 percent, followed by blank or donkey votes on 32 percent.

Tang was third on 19.2 percent and Albert Ho, of the Democratic Party, had 14.9 percent.

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