The Netherlands' Willem-Alexander became Europe's youngest monarch on Tuesday after his mother, queen Beatrix, abdicated and his country hailed the avowedly 21st-century king with a massive, orange-hued party.
Beatrix, 75, shed a tear before signing the act of abdication at the Royal Palace in Amsterdam, which was then witnessed by Willem-Alexander, 46, his Argentine-born Queen Maxima, 41, and members of the government.
A cry went up from the 25,000 crowd in the Dam, the main square opposite the palace where the signing was shown on giant screens.
Willem-Alexander, Maxima and Beatrix appeared before the crowds on the palace balcony, bedecked in roses and oranges.
"I'm happy and grateful to present you your new King Willem-Alexander," Beatrix said.
The new king expressed intense gratitude to his mother for "33 touching and inspiring years" and thanked the Dutch people for their support and trust, before leading a rendition of the national anthem.
Police escorted two republicans from in front of the royal palace after they brandished a large sign reading: "I'm not a subject".
They were escorted to an authorised protest area but police later apologised for detaining the anti-monarchists.
The king was to head with his queen and their three daughters, including their eldest, now Princess of Orange Catharina-Amalia, 9, to the neighbouring Nieuwe Kerk to be sworn in.
While already king, Willem-Alexander must also be sworn in rather than crowned, because church and royalty are separated in the Netherlands, at a ceremony before a joint session of the houses of parliament in the deconsecrated Nieuwe Kerk.
A who's who of royals-in-waiting, including Britain's Prince Charles, Spain's Prince Felipe and Japan's Prince Naruhito and his wife, Crown Princess Masako, are attending the ceremony.
Princess Masako is on her first trip abroad in nearly seven years, while Prince Charles also attended Beatrix's enthronement in 1980.
Willem-Alexander is the first Dutch king since 1890 and the first of a new wave of relatively youthful European monarchs.
"Beatrix has been queen for 33 years, our queen," Ruud, 49, told AFP on the Dam after the abdication, a tear in his eye.
"She was a stabilising factor and a symbol of our country. It's sad to see her go after all these years, a page in our collective history is turning.
Amsterdam's population is set to double with around a million visitors flooding the city's streets and canals to mark the abdication and enthronement.
Some people spent the night on Amsterdam's main square to ensure having a good view of the royal balcony, while partygoers from across the Netherlands descended on the capital, many carrying Dutch flags.
Over 10,000 police have been deployed in Amsterdam, with police saying they had arrested 70 people since Monday.
Authorities have closed off Amsterdam airspace to civilian aircraft for three days and issued strict orders prohibiting the use of drones, with rooftop snipers keeping a watchful eye.
The monarchy is popular in the Netherlands, but some question the cost of the royal household and republicans are seeking to get the king's 825,000 euro (million-dollar) tax-free salary reduced.
Queen Maxima attended a gala dinner at the Rijksmuseum on Monday, wearing a Valentino dress to the dinner that she had previously worn in 2008.
Dutch media said that this was a deliberate move to reduce the royal family's image of extravagance in times of economic belt-tightening.
While Beatrix was known for her formal court, Willem-Alexander has already said that he will not be a "protocol fetishist".
Beatrix's enthronement in 1980 was marred by violent protests and running street battles over a housing crisis that left the city looking like a war zone.
Anti-royalists this time have been allotted six locations in Amsterdam to stage protests. But only one has been booked by Republicans planning playful protests, including by wearing white.
Preparations for the day have been overshadowed by a rancorous debate about the event's official song, known as the Koningslied, which many considered ill-fitting, with its mix of traditional and rap music.
The nation will now sing the Koningslied as one on Tuesday evening, just before the royal family heads off on a water pageant behind Amsterdam's central train station.
Maxima is largely responsible for having made her husband popular after an allegedly boozy youth which earned him the nickname "Prince Pils".
Ever smiling, she has mastered the Dutch language and even taken a charity swim in Amsterdam's canals, endearing herself further in a country that expects their royals to be at once normal and regal.
Speaking ahead of the enthronement, Willem-Alexander said that "people can address me as they wish because then they can feel comfortable."
He stressed he wanted to "be a king that can bring society together, representative and encouraging in the 21st century".