Ailing for the last few months, Mr Singh fainted at his home and was rushed to the M S Ramaiah hospital where doctors declared him dead about 40 minutes later, a doctor who attended on him said.
"He was sweating profusely (when he was rushed to hospital). Cardiologist test was conducted for 40 minutes. He was not responding. He was declared dead around 11:30 am," Dr V R Anil Kumar, associate professor in the Department of Emergency Medicine at the hospital, told PTI.
Mr Singh, who headed the Congress-JDS coalition government from 2004 to 2006, represented Jewargi constituency seven times consecutively in the legislative assembly, and earned the title 'solillada saradara' (the invincible).
He is survived by wife Prabhavati, sons Vijay Singh (Bidar MLC), Ajay Singh (Jewargi MLA) and daughter Priyadarshini.
Elected twice to Parliament from Gulbarga and Bidar respectively, Mr Singh was also Karnataka Congress chief.
The Karnataka government declared a three-day state mourning and holiday for government offices, schools and colleges across the state today.
The body which was kept first at his city residence before being taken to KPCC office for public and party functionaries to pay their last respects, was flown to Bidar, from where it was taken to Jewargi before being shifted to Kalburgi for his last rites, officials said.
Known for his gentle demeanour, Mr Singh was considered an "Ajatashatru'' (with no enemy) in Karnataka's political circles.
Recalling Mr Singh's strong and vast administrative acumen, Mr Siddaramaiah in his condolence message said "the freedom and affection showed by him towards me as his deputy during the coalition government rule is unforgettable."
A veteran state Congressman, Mr Singh remained loyal to the party till the end and had served in the ministries of D Devaraj Urs, R Gundu Rao, Veerappa Moily and S M Krishna holding key portfolios, including Home and Public Works Department.
Leader of Congress party in Lok Sabha Mallikarjuna Kharge whose friendship with Mr Singh was well known, as they both are considered "inseparable twins", got emotional remembering his "friend".
"His death is a great shock to me, our friendship is over 50 years old. I feel like losing my elder brother," he said with tears in his eyes.
Several state leaders including BJP president B S Yeddyurappa and former prime minister H D Deve Gowda have expressed grief over Singh's death, terming it as great loss to the state.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)