Hacker outfit Anonymous has been taking down Greek government websites on the eve of a visit by German Chancellor Angela Merkel.
The visit is likely to spark angry protests from Greeks who are furious at the way the Germans are demanding ever stronger austerity measures.
Reports from various news agencies said that several government websites appeared to go down briefly late on Monday, including those of the Citizens Protection Ministry, the police and the Ministry of Justice.
Police did not say who was responsible for the attack, but Anonymous put its hands up in a series of Tweets.
The group criticised the huge security operation that police plan for Tuesday to contain protests. Anonymous compared the government to the military junta that ruled Greece from 1967 to 1974.
Trade unions and opposition political parties have called for mass protests to greet the German chancellor.
The Greek government is currently locked in negotiations with its international lenders on yet more spending cuts to clinch the next part of a $169-billion bailout to save the country from bankruptcy.