BRUSSELS, Brussels Capital Region - Philippe ascended the throne of Belgium after the emotional abdication of his father Albert II on Sunday, vowing to strive for unity in a nation divided by language and tradition.
The new sovereign, who at 53 becomes the seventh "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.
"I begin my reign with the will to serve all Belgians," he said in a first speech as king that brought a standing ovation in the federal parliament.
As cannons boomed out a 101-gun royal salute, Philippe said "Belgium's strength lies in giving meaning to our diversity."
"Let us all together show our country a new surge of enthusiasm."
Flanked by his popular wife Mathilde, the first Belgian-born queen, Philippe and their four young children were greeted with wild cries of "Long Live the King" by a sea of people massed outside the palace as they appeared on its balcony.
He was later to review troops and attend the annual National Day military parade.
Under sunny skies and a light summer breeze, flags fluttered across Brussels as the day of pageantry began with a thanksgiving mass in the cathedral marking the handover after Albert's 20 years at the helm of the fragile nation.
At a solemn abdication ceremony 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 then turned to his wife of 54 years, Queen Paola, to say: "As for the queen who constantly supported me in my task I would simply like to tell her 'thank you.'
"A big kiss", he added as she shed a tear.
"It is a new page for the monarchy," said Maximilien De Wouters, a student of 24 draped in the black, yellow and red national flag.
But worries persist that the shy and often awkward prince Philippe may lack the political skills of his father to maintain unity in a nation deeply divided between its Flemish- and French-speaking halves.
Mathilde, an outgoing 40-year-old who attended the celebrations dressed in white, is seen as his best asset in the couple's campaign to win the hearts of their 11.5 million people.
"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."
The monarchy more often than not is viewed as a rare symbol of Belgium's unity - along with its iconic fries and the national football team.
But while the French-speakers of the south remain largely royalist, Flemish-speaking Flanders, home to 60 percent of the population, has cooled.
There, the powerful separatist N-VA party favours a republic, or at least a royal as figurehead only.
"I am a fan of the royal family," said Cindy van Merheulen, 34, from Limburg in Flanders. "I want to welcome Philippe. Nearly all Belgians love the king; the problem is that those who are against shout louder."
Many fear that the separatist N-VA, the strongest party in Flanders, will make further gains in next May's general election.
In the last decades, severe tensions across the linguistic divide in a country that hosts key international institutions such as the EU and NATO, have seen it morph progressively into a federal state that devolves increasing powers to its language-based regions.
Albert helped steer the country through several crises and avoid break-up, notably playing a key role to end a 2010-11 crisis when the country endured a record-breaking 541 days without a government.