Belgium has sworn in Philippe as its new king after the abdication of his 79-year-old father Albert.
Philippe, 53, took the oath in the country's three official languages - French, Flemish and German - before the nation's parliament on Sunday.
Earlier, the 79-year-old Albert signed away his rights as the kingdom's largely ceremonial ruler at the royal palace in the presence of Prime Minister Elio Di Rupo, who holds the political power in this 183-year-old parliamentary democracy.
Flags fluttered across Brussels as crowds in the streets shouted "Long Live the King" in farewell to Albert.
"Belgium is modernising itself and it gives me joy," Albert said. He also called for continued "cohesion" between the nation's 6 million Dutch-speaking Flemings and 4.5 million French-speakers.
Under crystal chandeliers in a gilded hall at the royal palace, Di Rupo called Albert "a great head of state" and told the outgoing king, "You are closing an important page in the history of our country".
Early in the day, both Albert and Philippe mingled with the crowds under a royal blue sky following a Catholic ceremony that set off the festivities.
Albert announced his abdication plans less than three weeks ago.
He knows the politics of Belgium and Europe very well
Since the royal transition coincides with Belgium's national day celebrations, a military parade has already been planned.
Philippe will face a tough task in the coming months. The fractious nation, divided by language, holds parliamentary elections in June next year amid calls for even more autonomy for the language groups.
Many Dutch speakers have sought greater independence for Flanders in the north and wary of a monarchy seen rooted in the once powerful, but now poorer French-speaking Wallonia in the south.
Unlike his five predecessors, Albert tried to avoid politics as much as possible and Philippe is expected to do likewise.
Philippe has four children, including the oldest, Elisabeth, who at almost 12 becomes the country's first female heir to the throne ahead of her brothers.
The Oxford and Stanford-educated Philippe, has been trained as a Belgian air force pilot and led trade missions over the past two decades.
"He is a very wise person, a person who is very well prepared," said EU Commission President Jose Manuel Barroso, who attended the ceremony.
"He knows the politics of Belgium and Europe very well."
His wife Mathilde, who will be the first Belgian queen actually born in the country, is popular across the nation because of her natural style and charity work.