In her condolence letter to Sushma Swaraj's husband Swaraj Kaushal, Sonia Gandhi said: "I am shocked and deeply saddened at the sudden passing away of your beloved wife."
Gandhi said Sushma Swaraj was a lady of "extraordinary gifts" and her courage, determination, dedication and ability were manifest in every position she held.
"Above all, it was her warm personal qualities that brought a special lustre to her years in public and political life," she said.
Sushma Swaraj, 67, died on Tuesday night at the All India Institute of Medical Sciences (AIIMS) following cardiac arrest.
Gandhi said: "Sushma Swaraj made herself accessible to even the humblest person, established a warm rapport with people from all walks of life, and with her empathy and compassion gave Indian diplomacy a human face as she reached out to help every Indian citizen in distress.
"Sushmaji was a superb orator, a great parliamentarian and had a rare gift for friendship that won her affection and admiration across the political spectrum.
"In our many years together as colleagues in the Lok Sabha, we developed a warm personal relationship and I feel her loss greatly," she said.
Gandhi said Sushma Swaraj "faced her serious ailments with incredible fortitude. She has left us when she was far too young, when she had so much more to contribute to national life, and that makes her death even more tragic.
"At this time of grief, my thoughts and prayers are with you and your daughter Bansuri. May you find the strength to bear your cruel loss.
"You must draw solace from the fact that Sushmaji lived a life of honour, filled with achievement and contributions to the country she loved: respected and admired by all Ind. And she went as she had lived - active and engaged till the very end," Gandhi added.
( With inputs from IANS )