By Ilaria Polleschi

MILAN (Reuters) - Former Italian prime minister Silvio Berlusconi had open heart surgery at the age of 79 on Tuesday, an operation his doctors described as a success and which his supporters greeted as a sign that he could soon return to the political fray.

The center-right leader and media mogul was taken to hospital last week suffering from what doctors called a life-threatening cardiac deficiency. Berlusconi, still engaged in active politics, has worn a pacemaker for the past decade.

He underwent a four-hour operation at Milan's private San Raffaele hospital to replace a defective aortic valve.

"The operation went as planned, very well, without complications," said surgeon Ottavio Alfieri. "He is in intensive care now."

Berlusconi's younger brother, Paolo, told reporters outside the hospital, renowned for its cardiac unit, that Berlusconi had breathed normally throughout the operation.

"We were not particularly worried before. We are even more relaxed now," he said.

"His heart is very strong and in good shape, I am sure that he will overcome this trial and will come out of this the same as before, or even stronger and more determined than before."

The four-time premier has survived numerous diplomatic gaffes, legal brushes and controversy, including allegations he had sex with an underage girl and hosted orgies.

Family and supporters have kept a vigil at the hospital since he was admitted. One supporter erected a banner -- "Forza Silvio. Don't give up" -- while his girlfriend, 30-year-old Francesca Pascale, was pictured in local media wiping away tears while looking out from a hospital window.

"Francesca was emotional like all women. She is at his side, little by little his children will arrive too," Paolo Berlusconi said.

A hospital official said Berlusconi would stay in intensive care for 48 hours and would be under sedation until Wednesday.

In a post on Facebook, Berlusconi wrote on Monday: "Naturally I am worried, but I have been very much consoled by the outpouring of affection and support that have come from many sides, including from so-called political enemies."

Berlusconi's personal doctor, Alberto Zangrillo, had said last week that he was in very serious condition when he was taken to the hospital but was expected to recover fully from the surgery within a month.

Berlusconi's once-powerful center-right group of parties has become increasingly fragmented in recent years, mostly since he lost his grip on power in 2011 at the height of the euro zone debt crisis.

His Forza Italia (Go Italy!) party suffered mixed fortunes in local elections this month, with his candidate for Rome mayor finishing fourth.

Berlusconi was forced to take a back seat from the political scene when he was ejected from the Senate in 2013 following a conviction for tax fraud.

Last year, a court in southern Italy found him guilty of bribing a senator to switch factions in a move which helped topple a center-left government in 2008.

That court also banned him from holding any public office for five years.

(additional reporting by Valentina Consiglio; Writing by Silvia Aloisi; Editing by Richard Balmforth)