Ninety-eight-year-old Singh, the only officer of the IAF to be promoted to five-star rank, equal to a Field Marshal in the Army, was admitted to the Army's Research and Referral hospital this morning after he suffered a cardiac arrest, the defence ministry said.
"A team of doctors is monitoring his condition at Cardio- Thoracic and Vascular Centre of the Army hospital," it said in a statement.
Prime Minister Narendra Modi, Defence Minister Nirmala Sitharaman and the three Service chiefs -- Gen. Bipin Rawat, Admiral Sunil Lanba and Air Chief Marshal Birender Singh Dhanoa -- visited Singh at the hospital.
"We are all praying for the speedy recovery of Marshal of the Indian Air Force Arjan Singh. Doctors are doing their best," he said.
Sitharaman expressed the hope that Singh would recover at the earliest, adding his condition remained critical.
An icon in the country's military history, Singh had led a fledgling IAF in the 1965 Indo-Pak war when he was just a 44-year-old.
As Pakistan launched its Operation Grand Slam with an armoured thrust targeted at the vital town of Akhnoor in Jammu and Kashmir, he led the IAF through the war with courage, determination and professional skill.
The fighter pilot, who inspired the IAF despite constraints on the full-scale use of air combat power, was awarded the Padma Vibhushan, the second highest civilian honour, in 1965.
Born on April 15, 1919 in Lyallpur in Punjab in undivided India, his father, grandfather and great grandfather had served in the cavalry.
Educated at Montgomery, British India (now in Pakistan), he had joined the RAF College, Cranwell in 1938 and was commissioned as a Pilot Officer in December the following year.
Singh had led an IAF squadron into combat during the 1944 Arakan Campaign and was awarded the Distinguished Flying Cross (DFC) that year.
He was the IAF chief from August one, 1964 till July 15, 1969.
Field Marshals Sam Manekshaw and K M Cariappa of the Army were the two other officers with a five-star rank.
After his retirement from the air force, Singh was appointed as the India's Ambassador to Switzerland in 1971 and concurrently served as the Ambassador to the Vatican.
He was also the High Commissioner to Kenya in 1974.
Singh served as a member of the National Commission for Minorities and was also the Lieutenant Governor of Delhi. He was made Marshal of the Air Force in January 2002.
The fighter aircraft base at Panagarh in West Bengal was named in his honour on his birthday last year.