The Netherlands' Willem-Alexander became Europe's youngest monarch on Tuesday after his mother, queen Beatrix, abdicated and his country greeted their first king in more than 120 years with a massive orange-hued party.
Beatrix, 75, shed a tear before signing the act of abdication at the Royal Palace in Amsterdam, witnessed by Willem-Alexander, 46, his Argentine-born Queen Maxima, 41, and members of the government.
A cry went up from the 25,000-strong crowd in the Dam, the main square opposite the palace where the signing was shown on giant screens.
Willem-Alexander, Maxima and Beatrix appeared in front of the crowds on the palace balcony, bedecked with roses and oranges -- the symbol of the royal house -- before heading for the enthronement ceremony in the neighbouring Nieuwe Kerk.
Beatrix accompanied the king and queen's three daughters to the church, including their eldest, now Princess of Orange Catharina-Amalia, nine.
The Dutch monarch is sworn in before a joint session of the houses of parliament in the deconsecrated church, rather than crowned, because church and royalty are separated in The Netherlands.
The king entered the church at a stately pace with Maxima under an awning of fishing nets, an ancient tradition in the seafaring nation.
Before taking his oath, the king thanked his "dear mother" for the "many beautiful years during which she was our queen".
"I'm treading in your footsteps. I have a clear vision of my office. But no one knows what the future brings," he said.
"Wherever that path leads and however far it goes, I will carry your wisdom and warmth with me," he said.
The king swore "to preserve the independence and territory of the kingdom to the best of my ability ... so help me God."
Dutch animal rights activists have criticised his ermine-lined cloak, but Willem-Alexander noted that it is old and so no blood had recently been shed for it.
MPs and senators then swore an oath to the king, although 16 MPs have refused to do so, saying their allegiance is to the constitution.
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, attended the ceremony.
Princess Masako is on her first trip abroad in nearly seven years, while Prince Charles also attended Beatrix's enthronement in 1980.
Former UN secretary general Kofi Annan and International Olympic Committee head Jacques Rogge also attended.
-- 'A page in our collective history is turning' --
Police escorted two republicans from outside the royal palace shortly before the abdication 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.
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 estimated to have doubled with around a million visitors flooding the city's streets and canals to mark the abdication and enthronement.
Over 10,000 police have been deployed in Amsterdam, with authorities reporting 70 arrests since Monday.
Although the monarchy is popular in The Netherlands, some question the cost of the royal household and republicans are seeking a reduction to the king's tax-free salary of 825,000 euros (about $1.0 million).
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 around 100 republicans turned up for a protest at just one of the locations, an AFP correspondent reported.
Preparations for the day have been overshadowed by a rancorous debate about the event's official song, known as the Koningslied, which many consider inappropriate, 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 on the Ij river.
Around 200 boats will join the royal flotilla, including a large gay pride boat, as the Koningslied concert to be broadcast live from Rotterdam kicks off.
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 stressed he wanted to "be a king that can bring society together, representative and encouraging in the 21st century".