The Oxford- and Stanford-educated, trained air force pilot took the oath as the country's seventh king in a ceremony in parliament.

To warm applause, King Philippe, 53, promised to uphold the constitution.

Belgium has a constitutional monarchy in which the king plays a largely ceremonial role.

One of the duties the monarch does have is trying to resolve constitutional crises.

In a colourful ceremony topped off by trumpet fanfare and cannon-fire, Philippe took his oath in the country's three official languages - Dutch, French and German.

In his final address before signing a legislative act to step down on Sunday, 79-year-old Albert said his country must remain a "source of inspiration" to Europe.

He had earlier urged Belgians to give son their support, saying he was no longer well enough to carry on.

Albert also stressed his wish that Belgium - split between the Dutch-speaking north and the French south - remained united.

His resignation after nearly 20 years on the throne comes on Belgium's national day.

In an emotional ceremony at the royal palace in Brussels, the former monarch told his son: "You have all the emotional and intellectual qualities to serve our country well."

He thanked an audience of some 250 dignitaries and political leaders "for all that you have achieved during my reign".

Albert also thanked his wife, Paola, for the support she had given him during his reign, and was in turn thanked by Prime Minister Elio Di Rupo for his service to his country.

Mr Di Rupo holds the political power in the 183-year-old parliamentary democracy.

Albert then embraced his son and signed the official abdication papers, ending his reign.

King Albert exercised his authority in mediating between political leaders on the formation of a government during the 2010-2011 parliamentary stalemate, when Belgium was left without a government for 541 days after elections failed to find a clear winner.

Tensions between the Dutch-speaking and French communities sometimes run high, and the issue has brought down several governments, creating frequent political instability.

Respect for the royal family, though, is one of the few factors that crosses the communal divide.

King Albert's abdication comes only three months after Queen Beatrix of the neighbouring Netherlands vacated the Dutch throne in favour of her son Willem-Alexander.