Former Portugal and Benfica striker Eusebio has been taken to hospital in Poznan after falling ill on Saturday night.
The 70-year-old complained of feeling unwell in Opalenica, where he was accompanying the Portugal squad during their Euro 2012 campaign.
The Portuguese Football Federation said in a statement on their website: "National-team ambassador Eusebio felt unwell on Saturday when he was at the hotel with the national team in Opalenica."
The statement continued: "After an initial observation and contacting his doctor in Lisbon, he moved as a precaution to a hospital in Poznan for further investigations."
Eusebio, who underwent a heart operation in his homeland five years ago to prevent a possible stroke, was treated for bronchial pneumonia last December and has been admitted to hospital several times since.
He was seen in the stands alongside former Portugal captain Luis Figo on Thursday enjoying the team's 1-0 quarter-final victory over Czech Republic.
Eusebio played for nine clubs in a 22-year professional career but it was at Benfica where he forged his reputation as one of the game's greats, turning out for the club 614 times between 1960 and 1975 and scoring an astonishing 638 goals.
During his time at Benfica, Eusebio won 11 Portuguese league titles, five Portuguese Cup titles and the European Cup in 1962.
Despite being born in Mozambique, he was eligible to play only for Portugal as his homeland was a dependency of the European nation. He scored 41 goals in 64 international appearances between 1961 and 1973, and finished as the top scorer at the 1966 World Cup in England.