Philippe’s investiture came two-and-a-half weeks after King Albert, 79, announced he would abdicate after 20 years on the throne to make way for the 53-year-old crown prince.
Before signing a legislative act to step down on Sunday, Albert thanked his wife, who wiped away tears, and said his son had all the qualities to serve the country well.
“My final recommendation to all those gathered here is to work without rest for keeping Belgium together,” he said at the ceremony in the royal palace in Brussels.
The 183-year-old country is split across the middle, with many Dutch speakers seeking greater independence for Flanders in the north and wary of a monarchy seen rooted in the once powerful, but now poorer French-speaking Wallonia in the south.
“One king, two nations” was a headline in French language business daily L’Echo.
Outside the palace, a festive crowd gathered in readiness for the new king and queen to wave from the balcony in baking heat. Many carried Belgian flags, some with Philippe’s face on them.
“The new king is a bit of history. That doesn’t happen very often so we wanted to be here,” said Xavier De Graef from French-speaking Liege, clad in a Belgian football shirt, a flag and a wig in the red, yellow and black of the Belgian tricolour.
There were a few dissenting voices, including the N-VA party that wants Dutch-speaking Flanders to break away from Belgium and favours a republic.
“It leaves me cold. It doesn’t make the hairs on my arm stand up. This is part of my job as a lawmaker. Otherwise it just passes me by,” said Jan Jambon, its parliamentary chief.
The party has been particularly vocal in recent weeks about the need to reform the monarchy, but said it would not disturb Sunday’s pageantry. Far-right separatists Vlaams Belang said it would not attend the swearing in, but planned no protests.
However, Michiel Descheemaeker, a 21-year-old student who with friends was dressed in medieval costume, said he had come to protest against monarchies in general. “Kings belong in fairytales and that’s the only place,” he told Reuters television.
Fewer than half of people in Flanders believe Philippe will be a good king compared with two thirds in Wallonia, according to an opinion poll.
Belgian kings do plenty of handshaking and ribbon-cutting, but also appoint mediators and potential government heads to steer coalition talks after elections, no small task in Belgium.
Neighbouring Netherlands stripped its monarchs of involvement in politics last year. Queen Beatrix also stepped aside to allow her popular son Willem-Alexander to become king amid wild celebrations.
Philippe’s investiture was tagged on to festivities already planned for July 21, which is Belgium’s national day and also marked 20 years of Albert’s reign.
The Belgian government, mindful of budget savings it has forced on the public, has said this should help cap costs. Even royalist Belgians feel they know little about Philippe, who has appeared reserved in public, in contrast to his more outgoing father.
That included Brigitte Kittel, from Belgium’s 75,000 strong German-speaking community, with a flag on her cheek. “People in the German community like the king a lot,” she said. “We’re good Belgians. I don’t know what to think of the new king yet, I know too little about him.”