Belgium turns a page of history Sunday, ushering in its seventh king, Philippe, on a day of celebrations against a backdrop of continuing concern over the future of a divided nation.

At noon (1000 GMT), Philippe, 53, dressed in full military uniform, will take his oath of office in the country's three official languages -- French, Flemish and German -- before Parliament.

He takes over from his 79-year-old father Albert II, who formally abdicates at 0830 GMT, after 20 years on the throne, before an invited audience of some 250 dignitaries -- none of them foreign royals -- at the palace.

The new sovereign will be seated on a throne to become king but will have no crown nor sceptre as these are not part of Belgium's royal regalia.

In the first row of guests will be his wife Mathilde and their four young children, including the oldest, Elisabeth, who at almost 12 becomes at the same moment the country's first female heir to the throne ahead of her brothers.

Albert, who is abdicating due to his age and failing health, was not originally expected to become king but was forced to step up in 1993 following the sudden death of his elder brother Baudouin.

Worries persist that the shy and often awkward prince Philippe may lack the political skills and gumption of his father to maintain unity in a nation divided between its Flemish- and French-speaking halves.

Mathilde, a popular, outgoing 40-year-old who will be Belgium's first home-grown queen, is seen as his best asset in the couple's campaign to win the hearts of their 11.5 million people.

In a farewell address to the nation Saturday, Albert said that as both king and a father, his "very dear wish" was that Belgians offer their "support" to Philippe and Mathilde.

"They form an excellent couple at the service of our country," he said.

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 a republic, or at least a royal as figurehead only.

In the last decades, severe tensions across the linguistic divide in a country that hosts key global 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.

During his two decades at the helm, Albert II helped steer the country through several crises and avoid break-up. He played a key role to end its longest political crisis in 2010-2011 when it went a record-breaking 541 days without a government.

In his speech Saturday, he said his first wish as he stepped down was to see Belgium "retain its cohesion".

"I am convinced that maintaining the cohesion of our Federal state is vital, not only for our quality of life together, which requires dialogue, but also so as to preserve the well-being of all," he said.

Many fear that the separatist N-VA, the strongest party in Flanders, will make further gains in next May's general election.

With the country mindful of the need to tighten government spending due to Europe's economic crisis, Sunday's celebrations that take place on its National Day will be kept to around 600,000 euros, on a par with the annual July 21 event.

The celebrations, expected to draw hundreds of thousands to Brussels, kick off with a thanksgiving mass for the royal family early Sunday.

The rest of the day will feature royal appearances among the crowds, a military parade and fireworks.

There will be no foreign guests since Belgian custom is for there to be none, Prime Minister Elio Di Rupo's spokesman said last week.