Fiorentina captain and defender Davide Astori died of a sudden illness on Sunday morning at the age of 31, the Serie A club said, leaving Italian football in a state of shock.
Serie A announced that all of Sunday's matches, including the derby between AC Milan and Inter Milan, would be postponed.
"Fiorentina is deeply upset to have to announce that its captain Davide Astori has passed away, victim of a sudden illness," said Fiorentina's statement.
"Due to the terrible and delicate situation, and above all for the respect of his family, an appeal is made to everyone's sensibility."
Italian media said that Astori was found dead in his hotel room in Udine, where Fiorentina had been due to face Udinese at the Dacia Arena later on Sunday.
The Fiorentina staff became worried when Astori did not join his team mates for breakfast at around 9.30 am local time, local media reported.
Astori, who was in his 10th season as a Serie A player and made 289 top flight appearances, joined Fiorentina on loan in August 2015 and the club signed him outright one year later. He previously spent six seasons at Cagliari and one on loan at AS Roma.
He made 14 appearances for Italy between 2012 and 2017 and was in the squad at the Confederations Cup in 2013 where Italy finished third.
"It seems impossible. I'm in a state of disbelief and I cry with his family and all for Fiorentina. Goodbye captain," said former Italy Prime Minister Matteo Renzi.
In announcing the postponement of Sunday's games, Serie A said: "Shocked by the tragic news, the whole family of the Lega Serie A unites around the family of Davide Astori and Fiorentina FC."
AC Milan coach Gennaro Gattuso had asked before the announcement for all games to be called off. "I can't think of the derby after a tragedy like that," he said on Twitter.
The teams were already on the pitch and warming up for the lunchtime match between Genoa and Cagliari when news broke of Astori's death.
Fans applauded when Astori's picture was shown on the stadium screen and left in silence when it was announced that the game would not go ahead.
Other Serie A and European clubs and dozens of players sent messages of condolence as did federations such as European soccer body UEFA and global soccer body FIFA, and the global players' union FIFPro.
"On this day of tragedy, Juventus expresses its deepest condolences to Davide Astori’s family and Fiorentina," said Serie A champions Juventus.