Actor James Gandolfini died after suffering cardiac arrest, hospital officials in Rome say.
Dr Claudio Modini, head of the emergency room at the Policlinic Umberto I hospital, said Gandolfini arrived at the hospital at 10:20pm last night and was pronounced dead at 11pm after reanimation efforts in the ambulance and hospital failed.
He said a post mortem would be performed starting 24 hours after the death, as required by law.
It is understood he was in Italy to attend the 59th Taormina Film Festival.
Gandolfini played conflicted mob boss Tony Soprano in the groundbreaking HBO series from 1999 to 2007.
As news of his death spread, showbusiness tributes began to pour in.
Sopranos creator David Chase said: "He was a genius. Anyone who saw him even in the smallest of his performances knows that. He is one of the greatest actors of this or any time. A great deal of that genius resided in those sad eyes.
"He was my partner. He was my brother in ways I can't explain and never will be able to explain."
Lorraine Bracco, who played Tony's psychiatrist Jennifer Melfi in The Sopranos, said: "We lost a giant today. I am utterly heartbroken.
"I had the greatest sparring partner in the world, I had Muhammad Ali. He cares what he does, and does it extremely well."
Sad to hear about James Gandolfini . First met Jimmy back in '94.He was roommates in— Russell Crowe (@russellcrowe) June 20, 2013
NY with Lenny Loftin. Lovely man. RIP Jimmy
Brad Grey, a producer of The Sopranos, said: "Jimmy was one of the most talented, authentic and vulnerable actors of our time. He was unorthodox and truly special in so many ways. He had the sex appeal of Steve McQueen or Brando in his prime as well as the comedic genius of Jackie Gleason."
Joe Gannascoli, who played Vito Spatafore on The Sopranos, said: "Fifty-one and leaves a kid - he was newly married. His son is fatherless now ... It's way too young."
Satirist and director Armando Iannucci, who worked with the actor on the film 'In The Loop', paid tribute on Twitter.
He said: "People were surprised James Gandolfini was so different from Tony Soprano. That's because he was a great actor. Warm, gentle and funny."
People were surprised James Gandolfini was so different from Tony Soprano. That's because he was a great actor. Warm, gentle and funny.— Armando Iannucci (@Aiannucci) June 20, 2013
Actor and comedian Robin Williams said: "My thoughts and prayers go out to James Gandolfini's family. An extraordinary actor. RIP, Mr Gandolfini."
James Gandolfini. Unbelievably sad news. A fine man.— Steve Carell (@SteveCarell) June 20, 2013
Actress Bette Midler tweeted: "The great James Gandolfini passed away today. Only 51. I can't believe it."
Aerosmith guitarist Joe Perry tweeted: "James Gandolfini what a nice guy, so funny and sweet. He will be sorely missed. Our deepest condolences to his family."
Actress Rose McGowan posted a pictured of her and Gandolfini on Twitter, saying: "I am heartbroken about James Gandolfini. He was a gentle giant and great man. I love this picture, I loved him."
I am heartbroken about James Gandolfini. He was a gentle giant and great man. I love this picture, I loved him. pic.twitter.com/Y1wdbeUDfK— Rose McGowan (@rosemcgowan) June 19, 2013
Mia Farrow added on Twitter: "Awful awful news. James Gandolfini will be missed. He was a great actor. Just great."
And comedian Artie Lange said: "You will be missed greatly, James Gandolfini Maybe best actor ever on a TV series. Salute."