A statement issued by Madrid's Quironsalud University Hospital said 81-year-old Juan Carlos had a triple bypass procedure without any complications.

The operation had been planned following a check-up Juan Carlos received in June.

King Felipe VI, his only son, arrived at the hospital shortly after midday, accompanied by his mother Sofia, the former queen.

Arriving at the clinic late on Friday, Juan Carlos told reporters: "You'll see me on the way out."

The hospital said the former king was moved to the post-surgery unit after the operation.

Juan Carlos retired from public life in May. He abdicated in 2014 in favour of Felipe, ending a near 39-year reign.

Revered by many Spaniards, Juan Carlos was considered a key figure in Spain's transition from dictatorship to democracy following the 1975 death of General Francisco Franco.

Australian Associated Press