The open heart surgery on former Italian premier Silvio Berlusconi, needed to address a life-threatening condition, has started, the San Raffaele Hospital in Milan said in a Tuesday statement.
Doctors at the private clinic - where the 79-year-old checked himself in June 7, two days after suffering symptoms of heart failure - were due to instal a new valve to treat a "severe aortic insufficiency."
The hospital said the surgery started at 8 am (0600 GMT) and was expected to last about four hours.
"I am obviously worried. But I have also been very comforted by the very many messages of esteem, support and affection that I have received from all sides," Berlusconi wrote on Facebook on Monday.
Outside of the hospital, a few supporters of AC Milan, the football team owned by Berlusconi, unfolded a banner reading: "President don't give up."
On the eve of the operation - which carries a 2-per-cent mortality risk - the billionaire politician and businessman was visited by family, close aides and a Catholic priest, who took his confession and gave him the Holy Communion.
After the surgery, Berlusconi is expected to be kept in intensive care for a couple of days and will then have to follow a rehabilitation course lasting about a month, doctor Alberto Zangrillo said last week.
If doctors did not intervene, he would face a one-in-10 chance of dying before the end of the year, Zangrillo added.
Berlusconi, who turns 80 in September, was premier in 1994, 2001-2006 and 2008-2011. He was expelled from parliament in 2013 after a tax fraud conviction, but remains head of Forza Italia, a conservative opposition party.
He was admitted to San Raffaele twice in recent months. In December to replace a heart pacemaker, which he has had since 2006, and in March for cataract surgery. His medical history also includes a 1997 operation in the United States to remove prostate cancer.