A leading Hong Kong pro-democracy newspaper blamed Beijing on Thursday for a massive cyber-attack on its website, just days after a series of similar attacks on an online vote on electoral reform.

The website of Apple Daily, known for its critical stance on Beijing, suffered a blackout for several hours on Wednesday after what it described as a large-scale attack launched by sophisticated hackers.

The attack comes at a time of heightened tension in Hong Kong as pro-democracy activists worry that Beijing will renege on its promise of universal suffrage for the former British colony in 2017.

In a front page article headlined "We will never backtrack", the Apple Daily accused Beijing of orchestrating the cyber-attack.

"Apple Daily is attacked every day but this time the scale was unprecedented," said Cheung Ka-sing, chief executive of Next Media, which owns the newspaper.

"I don't want to speculate the motives of the hackers but I believe cyberattacks will continue to happen," the tabloid quoted him as saying.

When asked about the recent hackings, Chinese foreign ministry spokeswoman Hua Chunying said: "I'm not aware of the specifics. I'd like to point out that China is a victim of hacking from foreign countries."

The incident comes on the heels of a series of June 14 attacks on a website of "Occupy Central", a pro-democracy movement launched by local activists aimed at pushing China to allow universal suffrage in the semi-autonomous city, whose leader is currently chosen by a pro-Beijing committee.

- 'We can't be silenced' -

Occupy Central's "PopVote" website, built by the University of Hong Kong and Hong Kong Polytechnic University, is set to host an unofficial referendum on democratic reforms in the city on Friday.

Anson Chan, formerly number two in Hong Kong's government, accused Beijing of intimating pro-democracy activists through "downright blackmail".

"If Beijing is not worried about the referendum, why go to this lengths. Hong Kong cannot remain silent. I urge all Hong Kong people to turn up to vote," she said as she addressed Hong Kong's Foreign Correspondents' Club.

PopVote organiser Benny Tai said physical booths would be set up across Hong Kong for Friday's unofficial vote, although the project's servers may be shut down.

"Hong Kong people won't be scared away. We can't be silenced by this kind of attack. We have determination for universal suffrage," he said, adding that he expected up to 300,000 people to vote.

Beijing's cabinet last week published a controversial white paper reasserting China's control over Hong Kong, triggering angry protests in the city.

It was China's first ever policy document stipulating how Hong Kong should be governed, in what was widely interpreted as a warning to the city not to overstep the boundaries of its autonomy.

Hong Kong has been guaranteed semi-autonomous status and civil liberties under the "one country, two systems" arrangement reached by China and Britain before the handover in 1997, but there have been concerns that Beijing will exert more control over the city.

In March, the international Committee to Protect Journalists said media freedom in Hong Kong was "at a low point", citing self-censorship among reporters, financial and physical threats against the media and legislative steps that could hinder investigative reporting.

Paris-based Reporters Without Borders said in a recent report that Hong Kong's media independence was "in jeopardy", as China flexes its muscles to stifle critical coverage.