By his side was his wife of over 50 years, Queen Sofía, who looked elegant in a pale mint coat with a string of pearls gracing her neck, along with the Crown Prince Felipe and Princess Letizia.
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King Juan Carlos
Watching the official ceremony were the Crown prince and princess's daughters, the princesses Leonor and Sofia; Infanta Elena, Pilar and Infanta Dukes of Soria, along with the Presidents of the Congress and Senate, the Prime Minister and ministers, including approximately 150 institutional guests.
The royal couple will most likelykeep their titles of king and queen after Prince Felipe and Princess Letizia ascend the Spanish throne on 19 June, although the prince will be officially King at midnight on Wednesday.
On Friday the Council of Ministers suggested amendments to the decree which governs the titles of the royal family, proposing that the monarch and his consort will still be known as King Juan Carlos and Queen Sofía.
King Juan Carlos hugs his son the Crown Prince Felipe
Although Prince Felipe attended the official ceremony on Wednesday, it is thought that King Juan Carlos will not attend Thursday's coronation as it will be Felipe's big day.
The King decided to abdicate after a reign of nearly 39 years in early June. Widely credited with consolidating the country's democracy, his decision was announced to the nation by Prime Minister Rajoy, who urged Spaniards to show their gratitude to the 76-year-old monarch.
"He has been the best spokesman and representative of Spain in all corners of the world," said the Spanish leader.
Queen Sofía, King Juan Carlos, Prince Felipe, and Princess Letizia
He went on to pay tribute to the outgoing sovereign as "a tireless defender of our interests," adding: "I am convinced this is the best moment for change."
Mr Rajoy also expressed his confidence in the 46-year-old Prince, who will become Felipe VI, citing "his preparation, character and extensive experience in public affairs".