Several thousand people gathered outside Livorno's Armando Picchi stadium on Sunday to pay tribute to Piermario Morosini.
The Livorno midfielder, on loan with the club from Udinese, died on Saturday after collapsing on the pitch during his team's Serie B game at Pescara.
The fans left scarves and flowers outside the stadium, where a banner read: "You fought until the end, goodbye great Moro."
Livorno, the club Morosini had joined in January, issued a statement thanking everyone for the overwhelming support they have received.
It said: "The family, the girlfriend of Piermario and AS Livorno Calcio wants to thank all of the people that in this time of pain have remembered a great athlete and a special person."
Ernesto Randazzo, the player's agent, was still coming to terms with the tragic loss of the 25-year-old.
"I knew him for 13 years," he said.
"His aim was to make his stay at Livorno permanent. He had always been in very good condition and proof of that was all the medical check-ups he had undergone with all of the teams he had played for."
Morosini moved to Udinese in 2005 from Atalanta's youth academy before embarking on a series of loan spells elsewhere - first at Bologna, then Reggina, Padova, Vicenza and finally Livorno.
He made 18 appearances for Italy's U21s.
Udinese, the club that owned Morosini's sporting rights, have confirmed they will honour his memory by providing financial aid to the player's disabled sister.
A statement from Udinese read: "We are profoundly in pain for the premature death of Piermario Morosini.
"Through our non-profit organisation, 'Udinese for life', we will guarantee assistance to the sister of the unfortunate player, who has now been left on her own."
Morosini had been hit by a series of personal tragedies in his young life. The Bergamo-born player had lost both his parents before his 18th birthday.
Shortly after, Morosini's disabled brother committed suicide, leaving him alone with his handicapped elder sister.
An autopsy will be conducted on Monday to ascertain whether Morosini's cause of death was linked to a problem with the brain or the heart.
Pescara's local authorities have announced that an inquiry into Morosini's death will be opened after a parked car belonging to traffic police had initially blocked the ambulance's path into the stadium.