Real Madrid legend Alfredo Di Stefano is in an induced coma after a heart attack.
The 88-year-old is being treated at Madrid's Gregorio Maranon Hospital, club spokeswoman Marta Santisteban told The Associated Press.
A spokeswoman for Samur, Madrid's emergency service, told AP that an ambulance was called to Di Stefano at Juan Ramon Jimenez street, just north of the Bernabeu stadium, at 5 p.m.
She said paramedics found Di Stefano's heart had stopped beating and he had stopped breathing, but they managed to resuscitate him after 18 minutes.
A hospital spokesman later told AFP that Di Stefano was in a coma in the cardiac ward of the hospital.
A medical report from the hospital, which Madrid published on their official website, said: "The patient is currently in the coronary unit at the Gregorio Maranon Hospital. He is under sedation, with endotracheal intubation and artificial ventilation, and is being subjected to the relevant clinical trials.
"The patient is clinically and haemodynamically stable, but his condition remains serious."
Argentina striker Gonzalo Higuain, speaking after his team's 1-0 victory over Belgium in the World Cup quarterfinals, said: "Yes, I was told when I got to the dressing room. It's not good news for football and for those who know him.
"Alfredo was always very close to me and I have great respect for him. I send him my greetings and hope he will recover."
Former Argentina forward Di Stefano helped Madrid win five straight European Champions Cups, from 1956-60, and eight Spanish league titles.
He was voted European player of the year in 1957 and 1959 and retired at the age of 40 in 1966, ending a 21-year playing career.
Madrid appointed Di Stefano as honorary president in 2000 and named their new training complex after him six years later, with a statue erected in his honour in 2008.
Di Stefano underwent a quadruple aorta-coronary bypass with a pacemaker implanted in December 2005 after suffering a myocardial infarction.
Information from the Press Association was used in this report