(Adds Defence Ministry comment)

BEIJING, Oct 10 (Reuters) - Anonymous, the nebulous online activist group that uses hacking to further causes it supports, has threatened a major blackout of Chinese and Hong Kong government websites, and to leak tens of thousands of government email address details.

The group, under the banner of 'Operation Hong Kong' or '#OpHongKong' and '#OpHK' on Twitter, said on Friday it will launch a mass effort against Chinese government servers to bring down their websites via Distributed Denial of Service (DDoS) attacks on Saturday.

DDoS attacks attempt to cripple networks by overwhelming them with Internet traffic.

"Here's your heads up, prepare for us, try to stop it, the only success you will have will be taking all your sites offline," an Anonymous statement posted online said. "China, you cannot stop us. You should have expected us before abusing your power against the citizens of Hong Kong."

Demonstrations in Hong Kong have seen the use of tear gas, violent clashes and mass disruptions to business and traffic as people campaign for the right to democratically elect the Asian financial hub's leader.

Hong Kong's refusal so far to negotiate with protesters, and a police reaction that many labelled as heavy-handed, has sparked widespread condemnation that has now spread to Anonymous, which often campaigns for civil liberties by attacking people or institutions it sees as opponents of those rights.

"If this is true, it will show that the Chinese government is a victim of internet hacking," said Foreign Ministry spokesman Hong Lei at a daily news briefing. "China has consistently stressed our opposition to all internet hacking attack activities. We rebuke the acts of this organisation."

The Chinese government's Hong Kong Liaison Office also said its website had been attacked twice on Wednesday and Thursday, blocking visitors to the site for a time.

"This kind of internet attack violates the law and social morals, and we have already reported it to the police," it said, adding that the website was running normally again.

Among the websites Anonymous said it would target are those of China's Ministry of Public Security, the Ministry of Defence, Ministry of Justice and Hong Kong police.

"Prepping for massive DDoS attacks, Database dumps, etc... Will be destroying #China Government," wrote one Anonymous participant on Twitter.

China's Defence Ministry, in a statement sent to Reuters, said its website was subject to numerous hacking attacks every day from both home and overseas.

"We have taken necessary steps to protect the safe operation of the Defence Ministry website," it added.

The State Internet Information Office, China's internet regulator, declined to comment. The Ministry of Public Security declined to immediately comment by telephone. The Hong Kong Police Force was not available for immediate comment.

The Ministry of Justice said it was not aware of the threat from Anonymous, and that its website wasn't its responsibility to maintain.

The Legal Network Media Beijing Company, which maintains the Ministry of Justice site, said it had not had official notice about any attack, nor had it detected any attacks on the website so far.

"If there are future hacking attacks, we have confidence they can be resolved," said a technician at the company who gave his surname as Zhong. (Reporting by Paul Carsten; Additional reporting by Ben Blanchard and Beijing Newsroom; Editing by Jason Subler)