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 the govt members.

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.