Espanyol captain and defender Dani Jarque, 26, has died from heart failure while on a pre-season tour of Italy, the Primera Liga club have confirmed.
Jarque had trained in the morning but collapsed in his hotel room in Coverciano while talking to his girlfriend on the telephone in the early evening, Spanish media reports said.
"The Espanyol player died due to a systolic heart failure," the club said in a statement.
"(Club doctor) Doctor Cervera tried CPR on the player and used a defibrillator but said it was not possible to resuscitate.
"Within minutes, the first emergency ambulances arrived from Florence, and tried their defibrillators but again were unable to resuscitate."
Medical services tried to revive Jarque for an hour before pronouncing the player dead, the club said.
"Espanyol are distraught and wish to put themselves at the complete disposal of our captain's family," they added.
Club director German de la Cruz told radio station RAC1: "It's inexplicable. It's horrific.
"We are returning to Barcelona on Sunday but we can't come back with the coffin because they have to do an autopsy to confirm the cause of death.
"The players are destroyed. One minute he was there with them, and the next he's gone. It's terrible."
The club had been due to play friendlies against Bologna and Livorno in the coming days, on their Italian tour.
Jarque came up through the youth ranks at Espanyol, helping them win the Copa del Rey in 2006 and to reach the UEFA Cup final in 2007, and had just been made captain of the club for the new season.
Most Spanish clubs immediately expressed their condolences and leading footballing personalities have now done the same.
Raul said: "As captain and in representation of Real Madrid I want to show all our sympathy and to be by the side of the player's family.
"We want to give encouragement to all the Espanyol family at such a hard time, which has left us all filled with dismay and pain.
"Such a young lad, a great professional and sportsman, who suffered this has left us all very hurt. From here our most sincere sympathies."
Jarque's death comes two years after that of Sevilla and Spain defender Antonio Puerta, who suffered multiple organ failure in hospital, three days after a cardiac arrest during a league match against Getafe.
Madrid coach Manuel Pellegrini added: "It is news that you wish wasn't true. It's terrible, it already happened a couple of years ago to Sevilla and the truth is these things have an impact."
Fans have been flocking to Espanyol's stadium since the news broke on Saturday night and have been in mourning outside gate 21 - Jarque's squad number.
Former Espanyol coach Ramon Moya, who handed Jarque his debut against Rayo Vallecano, was deeply shocked.
"I feel like I have lost a son," he said. "Jarque was a footballer, but he was also my friend.
"This situation is very difficult for a person.
"It was as if he was family. I knew him for many years. I gave him his debut when he was a very young footballer and I shared many moments with him.
"It's very difficult to explain how I feel at the moment in words."
Another former coach, Ernesto Valverde, insisted Jarque had never had any medical problems previously.
"After hearing the news I still haven't taken it in," said the current Villarreal boss. "Above all because of how Jarque was.
"He was a player who was super strong, super healthy and very important in the dressing room.
"We got goosebumps when we heard the news.
"He was a very strong person but life is like that and it gives you terrible surprises.
"Everyone is very affected by the news, as is normal in these type of situations.
"He had his whole life ahead of him."
Valencia players sported black armbands in the second half of their friendly against Arsenal and held a minute's silence after learning the news during the interval.