Former prime minister Manmohan Singh was admitted to the All India Institute of Medical Sciences in the nation's capital on Sunday evening after he complained of uneasiness, sources said.
Singh, 87, is under observation at the cardio-thoracic ward of the premier hospital, they said.
"All his parameters are fine. He is under observation at the AIIMS," a source close to him said, adding that he was taken to the hospital after he complained of "uneasiness".
He was admitted under Dr Nitish Naik, a professor of cardiology at AIIMS, around 8.45 pm.
Singh, a senior leader of the Congress, is currently a Member of Parliament in the Rajya Sabha from Rajasthan. He was the prime minister between 2004 and 2014.
In 2009, Singh underwent a successful coronary bypass surgery at the AIIMS.
As the news came out, a number of leaders expressed concern over his health and wished him a speedy recovery.
Rajasthan Chief Minister Ashok Gehlot said, "Much worried to know former PM Manmohan Singh ji has been admitted to AIIMS. I wish him speedy recovery and pray for his good health and long life."
Senior Congress leader Ahmed Patel tweeted, "Praying for good health of Doctor saheb. We are sure he will be fit and fine at the earliest."
Rashtriya Janata Dal leader Lalu Prasad Yadav also tweeted, "Saw the news of Dr Manmohan Singh ji being admitted to hospital. My sincere prayers for the speedy recovery of Manmohan Singh ji, the humble, intelligent, scholar, true gentleman and one of the best PM."
His son and RJD chief Tejashvi Yadav said, "Wishing for the speedy recovery of respected Dr Manmohan Singh ji."