"All his parameters are fine. He is under observation at the AIIMS," a source close to him was quoted as saying by news agency PTI, 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 the All India Institute of Medical Sciences (AIIMS), around 8.45 pm.
Singh, a senior leader of the opposition 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.
Delhi Chief Minister Arvind Kejriwal said that the entire India is praying for former Prime Minister Manmohan Singh's health, as he was admitted to the All India Institute of Medical Science (AIIMS) here after complaining of chest pain.
"Deeply concerned about Dr Manmohan Singhji's health. Hope he makes a full recovery soon. All of India is praying for our former PM," Kejriwal tweeted.
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."
RJD 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 ManmohanSingh ji."