Papandreou said he was shocked by the bomb attack in the center of Oslo and the mass killings on Utoya, which cost the lives of innocent people and injured many others.
He said the description of the murderous attack against so many young people at the Labour Party's youth camp was terrifying.
Papandreou said that he fully shared the pain of the Norwegian people, who suffered such a violent terrorist hit, and conveyed the Greek government's and people's deep support.
The Greek premier further condemned such "heinous acts of violence" regardless of where they originate or their motives. "Our democratic societies and our culture cannot tolerate and will not tolerate methods and actions of terror and violence".
"In this difficult hour, we remember the solidarity you manifested in difficult times for the Greek people, when we in Greece sought refuge in Norway and found your support and
your strong voice for democracy and justice in our country," the letter continued, adding that Greece remains devoted to the long-standing friendship between the two peoples "and our common struggle against terrorist actions".
"We stand at the side of Norway and its people in these difficult moments," Papandreou concluded.