Thousands of Spaniards put aside their World Cup misery to line the sun-splashed streets, yelling "Long live the king!" as the newly-proclaimed Felipe, 46, and his glamorous Queen Letizia, 41, waved from an open-topped, black Rolls Royce.
A tall, former Olympic yachtsman, Felipe faces the task of polishing the image of a monarchy tarnished by scandals and winning over a country wearied by recession and political corruption.
Swearing his oath in parliament in a dark blue military uniform, Felipe promised "a renewed monarchy for new times", after scandals that tainted the reign of his abdicated father, Juan Carlos.
Applause and cries of "Long live the King!" filled the chamber as he finished his speech and turned to kiss Letizia, who wore a white knee-length dress and coat.
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After the swearing-in the king stood and waved from the car, flanked by guards on horseback with silver helmets and breastplates winking in the sun, during a drive with his wife through central Madrid to the old Royal Palace where a crowd of thousands was waiting.
Felipe and Letizia -- a former television newsreader -- appeared on the balcony of the Royal Palace with their blonde, blue-eyed daughters eight-year-old Leonor, who is now heiress to the throne, and Sofia, seven, and waved to cheering crowds below.
"We have lost the World Cup but that doesn't matter. It is a new day and a new king. We have to celebrate," said Eduardo Chaperon, a 24-year-old economist waving a Spanish flag and wearing a novelty inflatable crown in the street.
Not everyone joined in the party though.
Protests by campaigners who want Spain to be a republic broke out after Juan Carlos announced his abdication on June 2. Police banned a similar protest called by activists for today.
"It is shameful. It is a breach of our freedoms. What kind of democracy is this?" she said. "We spend a lot of money on all this but it doesn't serve Spain at all," she said of the royal family.
Police closed off city-centre avenues and snipers deployed on roofs in a 7,000-strong security operation for the royal festivities.