Former prime minister Manmohan Singh was admitted to the AIIMS here 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.
He was admitted under Dr Nitish Naik, a professor of cardiology at the All India Institute of Medical Sciences (AIIMS), around 8.45 pm.
"He is under observation," one of the sources said.
Singh is a senior leader of the opposition Congress and currently represents Rajasthan in the Upper House of Parliament. He was the prime minister between 2004 and 2014.