"Long live Spain! Long live the king!" cried early crowds outside parliament, where Felipe, a tall, 46-year-old former Olympic yachtsman, will be sworn in hours after he legally assumed the throne at midnight.
As police helicopters hovered above, officers closed city-centre avenues and snipers deployed on roofs in a 7,000-strong security operation for the royal festivities.
Felipe leads a royal family tarnished by scandal after the 39-year reign of his 76-year-old father Juan Carlos, who signed his act of abdication with tears in his eyes at the Royal Palace the day before.
"It feels like the end of a cycle," said Jose Antonio Gomez, who runs a soft drinks stall outside the old Royal Palace in central Madrid, where the new royals were to appear before the crowds on a balcony.
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"The Spain team were a disaster and have to renovate," said Gomez, after defending champions Spain crashed out of the World Cup in the first stage after a shock 2-0 loss to Chile.
"Dethroned!" declared the front-page of the Barcelona-based daily El Periodico of Spain's World Cup ejection. "Abdication!" blared sports daily Mundo Deportivo.
Thousands of red and yellow Spanish flags fluttered through the city, flying from buses and even covering police barricades. Pots of white flowers were sprinkled through the city.
Shops scrambled to sell T-shirts and fridge magnets commemorating the new generation of royals.
Felipe received the red silk sash of the military forces' captain general from his father before heading to parliament where he will enter through doors flanked by statues of lions and covered by a giant red canopy with the state coat of arms.
With his elegant 41-year-old wife Letizia, a former television news presenter and the granddaughter of a taxi driver, the king will then be driven from parliament through the streets of Madrid.