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Philippe I took over the throne after the emotional abdication of his father Albert II.
King Philippe I has taken the reigns as the seventh king of Belgium, succeeding his father Albert II who abdicated the throne.
Philippe, 53, took his vow in the country's three official languages - Dutch, French and German - in a ceremony in parliament.
The Oxford and Stanford-educated king will have the difficult task of keeping the fractured country united.
Belgium is largely split between the 6 million Dutch-speaking Flemings in the north and 4.5 million French-speakers in southern Wallonia.
The country holds parliamentary elections in June 2014 amid calls for even more autonomy for the language groups.
Belgium's royals are mostly ceremonial but they do play a key role in government by appointing mediators and potential government heads to steer coalition talks after elections, reports the Guardian.
The BBC's Chris Morris reports that along with the nation's soccer team, "the monarchy is often regarded as one of the few institutions that actually holds Belgium together."
The evening before his son was to take over, Albert, 79, made an emotional appeal to the people of Belgium.
"Today, it is with emotion that I speak to you one last time as king," said Albert.
"You will ask me, at the moment when I hand over my duties, what are my wishes for the future. My wish is that Belgium keeps its cohesion."
Philippe returned to that theme in his speech on Sunday.
"Time and again we find the balance between unity and diversity," King Philippe said.
"Belgium's strength is precisely that we make room for our differences."