"It was with the deepest sense of horror and sorrow that I learnt of the tragic demise of Madam Benazir Bhutto," Singh wrote in the condolence register kept at the Pakistan High Commission here.
"In her sad and untimely demise, Pakistan and South Asia have lost an outstanding political leader who was passionately committed to the cause of moderation, democracy and, peace and friendship between our two countries," he wrote.
"I convey my heartfelt condolences to the members of the bereaved family and the people of Pakistan," the Prime Minister wrote.
Singh earlier said in Panaji that the incident was a reminder of what terrorism can do and underlined the need for a joint fight against the menace.
"This great tragedy once again is a reminder to all of us of the great harm that terrorism can do to the peace, prosperity and well being of the region," he said.
"We all have obligation to work together to deal with this menace, which threatens the ways of civilised people living all over the world," Singh added.