Sweden’s King Carl XVI Gustaf is doing well after undergoing a planned surgical procedure "in the heart area," the palace said Monday.

The surgery "went according to plan and the King is doing well," the royal household said. "The king thanks for all the support and the warmth shown to him and feels grateful for Swedish healthcare."

The official announcement did not elaborate on the nature of the surgery. The king is not known to have had surgery for a any serious ailment before.

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After the surgery, the 76-year-monarch will take a rest period, meaning that his planned commitments this month and in early March will be pushed back to later this spring.

The king ascended the throne on Sept. 15, 1973 and is the longest reigning monarch in Sweden. Later this year, the king will celebrate his 50 years on the throne.

The royal family is hugely popular in Sweden. The monarchy enjoys widespread support despite the egalitarianism that otherwise characterizes society in the Nordic country.