Doctors decided to perform the surgery, which was due to take up to eight hours, after Singh complained of chest pains earlier in the week.
The procedure began at 7:15 am (0145 GMT) and doctors were upbeat about the prime minister's prognosis.
"We expect the prime minister to be up on his feet in the next couple of days in the intensive care unit," Dr. Sudhir Vaishnav of Mumbai's Asian Heart Institute said, adding Singh should be back at work within "two to four weeks.
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"I don't think the nation should worry (about the premier's health)," he told India's NDTV network.
The operation was being carried out at the premier state-run All India Institute of Medical Sciences in New Delhi.
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A high security ring was thrown around the hospital with gun-toting security personnel keeping a strict watch.
A team of 11 doctors from the Asian Heart Institute flew from Mumbai for the operation, said Vaishnav.
A hospital source earlier told AFP doctors were "looking at the removal of two