Nicknamed the Black Panther, Eusebio was regarded as one of the best players of all time and helped Portugal to third place at the 1966 World Cup, after scoring nine goals in the tournament in England.
The Mozambique-born striker made his name at club level with Benfica, winning 11 league titles during a 15-year spell there and two Golden Boot awards as Europe's leading goalscorer.
He also notched 41 goals in 64 appearances for Portugal.
Chelsea manager Jose Mourinho, speaking ahead of his side's 2-0 win in an FA Cup third round tie at Derby yesterday, said he was more than a footballer in Portugal and his feats will never be forgotten.
"He's unique in our country," Mourinho said.
"He's one of the greatest in the history of football, but for our country he's much more than that.
"Irreplaceable - his place in the history of Portuguese football, but more than that in the history of our country. He's a man that doesn't belong to Benfica, doesn't belong to a club, he belongs to a country and I prefer to say that people like him are immortal, because their history and their legacy remains forever.
"So of course I'm very sad, especially my assistant Silvino [Louro] because Eusebio was his coach for a long time, but I'm not so sad because I really feel that he's immortal for our country."
Benfica were unable to confirm details of Eusebio's death but reports in Portugal say he died of a cardiac arrest in the early hours of yesterday morning. He had been in poor health for some time and was hospitalised during Euro 2012 after suffering a heart scare in Poland.
The Portuguese Football Federation (FPF) said: "Portugal is mourning. Eusebio, the King of Portugal's 1966 team and the eternal symbol of the country, national team and Benfica passed away."
The Portuguese government declared three days of national mourning and many fans laid flowers at an iconic statue of him erected next to Benfica's Luz stadium.
Current Portugal captain Cristiano Ronaldo took to Twitter to remember the star. He wrote: "Always eternal #Eusebio, rest in peace."