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Siblings Queen Sofia of Spain, King Constantine II of Greece and Princess Irene of Greece and Denmark stood together as they remembered their father, who died on 6 March 1964.
Queen Sofia, 75, was supported by her three children Prince Felipe, Princess Cristina and Princess Elena of Spain, as well as Felipe's wife, Princess Letizia.
King Constantine and his wife Queen Anne-Marie were also joined by their children, who flew to Athens for the significant occasion.
London-based Prince Pavlos of Greece and his wife Princess Marie-Chantal brought their five children, Princess Maria Olympia, 17, Prince Constantine Alexios, 15, Prince Achileas Andreas, 13, Prince Odysseas-Kimon, nine and five-year-old Prince Aristidis-Stavros.
During the service the royals laid wreaths on the tomb of King of Paul, who is buried next to his wife Queen Frederica, including Princess Letizia and Prince Felipe, whose wreath also bore the name of their two daughter Leonor and Sofia.
The memorial service was held 5 km north of Athens's suburbs, at Tatoi Palace, the Greek Royals' former summer palace. The estate was abandoned in 1973, although many graves, including King Paul I's, remain in the Tatoi Royal Cemetery which is located in a large wooded area at the south end of the estate.
Despite the sombre occasion, the large royal gathering offered the opportunity for some rare family photos.
Princess Marie-Chantal, a keen user of Instagram, posted a picture of herself with her sister-in-law Princess Alexia and Spanish Princesses Cristina and Elena.
"My two favorite Spaniards and one Greek ❤️❤️," posted Marie-Chantal, closely followed by a group shot which also included Prince Nikolaos and his wife Princess Tatiana.