Willem-Alexander has been sworn in as king of the Netherlands following the abdication of Queen Beatrix.
He became the country's first king since 1890 when his mother signed an abdication deed after 33 years on the throne. Huge crowds of orange-clad partygoers are in Amsterdam to pay tribute.
Now known as Princess Beatrix, the former queen maintained a recent tradition of handing over power to a new generation.
Wearing the royal mantle, the new king swore to uphold the constitution at a colourful enthronement ceremony in the Nieuwe Kerk, a decommissioned church, before a joint session of the Dutch parliament.
"I swear that I shall defend and preserve the independence and territory of the state with all my powers," he said.
"That I shall protect the general and individual freedom and rights of all my subjects and shall use all available means granted to me by law for preserving and promoting general and individual prosperity as I befitting of a good king .... So help me God almighty."
Crowds in the square outside cheered as the announcement of his inauguration was made from a balcony overlooking the square amid trumpet fanfare.
The queen, 75, announced her intention to stand down in January, saying her son was ready to reign and that it was time for the throne to be held by "a new generation".
She formally relinquished the throne at a short ceremony in the Royal Palace on Tuesday, signing a statement transferring the monarchy to her son "in accordance with the statutes and the constitution of the Kingdom of the Netherlands".
There were huge cheers from the crowds outside in Dam Square, who were watching the ceremony on giant television screens, as she, her son and his wife Maxima, 41, signed the deed of abdication.
Shortly afterwards, the three emerged on a balcony above the square.
Princess Beatrix told the crowds: "I am happy and grateful to introduce to you your new king, Willem-Alexander."
The three held hands as the national anthem was played, before the new king and queen's three young daughters were brought out to wave at the crowds.
Their eldest daughter, Catharina-Amalia, has become Princess of Orange and is now first-in-line to the throne. She is aged nine.
Willem-Alexander, 46, has said he wants to "be a king that can bring society together, representative and encouraging in the 21st Century".
House of Orange-Nassau
He is the seventh monarch from the House of Orange-Nassau, which has ruled the Netherlands since the early 19th Century.
Under Dutch law, the monarch has few powers and the role is considered ceremonial.
He or she is expected to be politically impartial, co-sign acts of parliament, help with the formation of new governments and to undertake state visits.