After his father Albert 11's abdication, Crown Prince Philippe ascended to the Belgian throne amid fanfare and festivity and vowed to prevent the breakup of a nation split by language and history.
The new monarch, who at 53 becomes "King of The Belgians," was sworn into office by the joint houses of parliament shortly after Albert, aged 79, stepped down citing age and failing health.
"I swear to abide by the constitution and laws of the Belgian people, to maintain national independence and the integrity of the land," Philippe, dressed in full military uniform, said in the country's three languages -- French, Flemish and German.
President Barack Obama sent his best wishes to both the old and the new kings of a longtime ally that hosts key global institutions, the EU and NATO.
With political tension on the rise between the country's two main regions -- Dutch-speaking northern Flanders and the French-speaking south -- the monarch is a unifying force in the country of 11.5 million at the heart of Europe.
"I begin my reign with the will to serve all Belgians," Philippe said to a standing ovation in his first speech as king.
"Belgium's strength lies in giving meaning to our diversity," King Philippe added as cannons boomed out a 101-gun royal salute.
But the head of the country's powerful northern separatist movement, the N-VA, snubbed the swearing-in and supporters refused to applaud.
Under sunny skies and a light breeze, flags fluttered across Brussels, which threw a mega-party for the new ruling couple that brought half a million people into the streets.
Flanked by his popular wife Mathilde, Belgium's first native queen, Philippe and their four young children faced a chorus of "Long Live the King" from 10,000 well-wishers when they turned out on the palace balcony.
"In another country I'd be a republican but here our kings play their role to perfection," said Olivier Leleux, a 40-year-old finance expert.
One of the royal stars was the couple's oldest daughter Elisabeth, who at almost 12 and dressed in red became the first female to be next in line to the throne due to changes in legislation.
Stepping down in the royal palace's grandly chandeliered throne room, Albert stressed the need for the country's leaders "to work tirelessly in favour of Belgium's cohesion".
His voice breaking with emotion, he turned to Queen Paola, his wife of 54 years, and said: "I would simply like to say 'thank you ... A big kiss".
Paola, who is 75, shed a tear.
Belgium has fretted since Albert announced he was abdicating that the shy and often awkward Philippe, a trained fighter pilot who studied at Oxford and Stanford, might lack the political skills of his father to maintain unity in the divided nation.
But throughout the daylong celebrations, that included the National Day military parade and several royal walkabouts, Philippe appeared calm and at ease.
"We have had a very wonderful day," Philippe said at an evening appearance on the palace balcony where he and Mathilde exchanged embraces and pecks on the cheek.
"Thank you for your support, thank you for your confidence. Let's be proud of our beautiful country," he added.
Charismatic 40-year-old Queen Mathilde is seen as his best asset in taking the baton from his father, a jovial well-liked character with a popular touch.
"Philippe, you have the heart and the intelligence to serve our country very well," Albert said in his abdication speech. "You and your dear wife Mathilde have all our confidence."
Along with its iconic fries and the national football team, the monarchy is viewed as a rare symbol of Belgium's unity and Belgium football captain Vincent Kompany paid tribute to the royals after he stepped down.
"Our 'Low Country' deserves a real party, north to south, east to west! Good day to King Philippe and the radiant new Queen Mathilde. I wish you a positive reign and your father a fine rest," wrote Kompany, himself nicknamed Vince the Prince by fans of his club Manchester City.
Though French-speakers in the south remain largely royalist, Flemish-speaking Flanders, home to 60 percent of the population, has cooled to the king. There, the N-VA party favours a republic or, failing that, a mere figurehead monarchy.
The test will come in next May's general elections, where the N-VA, the strongest party in Flanders, may see further gains though the country has morphed into a federal state devolving increasing powers to its language-based regions.