Singh wrote a letter to Norwegian Prime Minister Jens Stoltenberg condoling the loss of lives in the violence.
"Please accept on behalf of the Government and people of India and on my own behalf my deepest condolences at the loss of so many innocent lives and injuries to scores of others in the senseless acts of violence in Oslo and the island of Utoya on 22 July, 2011," Singh said in the letter.
"These incidents have shocked us and we condemn them. They are a sad and grim reminder of the threat that extremist violence poses to open and democratic societies like Norway and India, and the need for all of us to unite in combating all forms of extremism," the Prime Minister said.
Singh assured Stoltenberg and the people of Norway of India's solidarity and support at this tragic hour.
"Please accept our heartfelt sympathy and condolences which I also request you to convey to the families of the bereaved," he said.