Rome - Italy international defender Davide Astori died in his hotel room during the night of Saturday to Sunday, his club Fiorentina said on Sunday.
"Fiorentina are profoundly shocked to have to announce the death of captain Davide Astori after a sudden illness," the Serie A club said on Twitter.
The 31-year-old, who leaves behind a two-year-old daughter, was in Udine to prepare for Sunday's match against Udinese.
Astori died of "natural causes" after a "cardiac circulatory collapse" - a type of heart attack - Antonio de Nicolo, a Udine magistrate was quoted in Italian media as saying.
The magistrate added that it was "strange that such a thing happened without any warning symptoms in a profession which is so closely monitored".
Astori's body was taken to a hospital in Udine for an autopsy.
Fiorentina's game was called off in the morning. The early kick off between Genoa and Cagliari, one of Astori's former clubs, was abandoned when players learned the news while warming up on the pitch.
The Italian league then postponed the other five Serie A matches scheduled for Sunday, including the Milan derby.
Astori began his career at AC Milan, also playing for Cagliari and Roma before joining Fiorentina in 2015. He played 14 times for Italy.
Former team-mates, coaches and teams were amongst those taking to social media to express their grief.
Gianluigi Buffon, the Juventus captain, called Astori "a great man".
"Farewell dear Asto", wrote Buffon in a post on Instagram accompanied by a photo of the two players exchanging pennants before a game between their clubs.
Buffon said he did not usually make a habit of expressing his feelings about others.
"I want to make an exception to my rule because you have a young wife and others close to you who are suffering, but mostly because your little girl deserves to know that her dad was a good man... a great man," Buffon wrote.
Radja Nainggolan, a Belgian international who played with Astori at two clubs, posted a photo of the two of them joyously celebrating a goal at Cagliari.
"A great player, but an even bigger man, so many battles fought together in Cagliari before coming to Rome, I cannot believe it," he wrote.
Astori's formative club Milan joined in.
"A man who loved football and who grew up with us. AC Milan is shocked by the tragic death of Davide Astori," the club's official account tweeted in English.
Matteo Renzi, a former mayor of Florence and now leader of the Democrat Party in Sunday's Italian election, tweeted: "This seems to me impossible. I don't believe it and I cry with the family and everyone at Fiorentina. Ciao capitano."