“Iker Casillas suffered an acute myocardial infarction during morning practice this Wednesday, at the PortoGaia Sports Formation and Training Centre, in Olival,” Porto said in a statement sent to CNN.
“The work session was promptly interrupted so that assistance could be given to the FC Porto goalkeeper, who is at this moment at the CUP Porto Hospital. Casillas is fine, stable and with his cardiac problem resolved.”
Casillas won the World Cup with Spain in 2010 and spent 16 seasons with La Liga giant Real Madrid, winning three Champions League titles and five La Liga crowns.
The 37-year-old moved to Porto in 2015 where he won the Portuguese title last season.
Widely considered one of the greatest goalkeepers of all time, Casillas made his professional debut as a teenager for Real Madrid and went on to make 725 appearances for the club.
He was an integral part of the Spanish national team as it dominated international football for four years, captaining his country to victory at Euro 2008, the 2010 World Cup and Euro 2012.
Casillas signed a new two-year contract with Porto in March and has helped the club qualify for the knockout stages of the Champions League for the past two seasons.
He currently holds the record of most career Champions League appearances with 177.
Real Madrid captain Sergio Ramos, who played with Casillas for 10 years, and current Real keeper Thibault Courtois sent messages of support to the Porto star.