Iker Casillas tweeted that he was "feeling strong" after suffering a heart attack at Porto's training ground on Wednesday morning.
The Portuguese club confirmed the 37-year-old goalkeeper was rushed to hospital after collapsing during a morning training session.
Casillas remains under observation but is now in a stable condition following surgery.
He later tweeted: "Everything under control here, it was a big scare but I'm feeling strong. Thanks a lot for all your messages and your support."
Casillas came through Real Madrid's academy and spent the bulk of his career there before joining Porto in July 2015.
His 725 matches between 1999 and 2015 but him behind only his former team mate Raul, with 741, on Real's all-time list. During that time he won five La Liga titles and three Champions League crowns.
He captained Spain to their sole World Cup win in 2010.
Real Madrid tweeted soon after the news broke: "Real Madrid would like to offer all our support to our beloved captain Iker Casillas. Throughout his professional career, he always overcame the hardest challenges in order to bring glory to our club ...
"He has taught us to never give up and has shown us numerous times that the best way to win is to remain as strong as possible when times are tough."
A tweet on the Spain team's official account said: "All the family at the national team and the Spanish football federation are with you Iker Casillas. Get well soon."
Additional reporting by Reuters.
