Ex-Atletico star Paulo Futre said: “Sad day. My deepest condolences to the family of my dear friend Marcos Alonso.
“Rest in peace, Pigeon.”
His ex-club Rayo Vallecano added: “From Rayo Vallecano, we want to convey our deepest condolences to the family and friends of Marcos Alonso Pena.
“He was a former coach of the Club, who passed away at the age of 63.”
Barcelona tweeted: “FC Barcelona would like to express its deepest condolences for the passing of former Barca player and father of a first-team player Marcos Alonso Pena.
“All of our strength to Marcos Alonso and his entire family. Rest in peace.”
His five-year club Barcelona expressed their “deepest condolences” before president Joan Laporta observed a moment of mourning during a press conference.
Before joining Atletico Madrid, Alonso Pena began his career in the late 1970s with Racing Santander. Later, he joined Barcelona, where over the course of five years, he played 124 games and scored 28 goals.
Later in his career, the Spanish winger rejoined Atletico and Santander before quitting in 1991. Alonso, who passed away, also had 22 appearances and one goal for Spain at the international level.