Denmark’s prime minister proclaimed Frederik X King on Sunday after his mother, Queen Margrethe II, formally signed her abdication, with huge crowds turning out to rejoice at the throne passing from a beloved monarch to her popular son.

Margrethe, 83, is the first Danish monarch to voluntarily relinquish the throne in nearly 900 years.

Thousands of people gathered outside the palace where the royal succession was taking place, the mood jubilant as the Nordic nation experienced its first royal succession for more than half a century, and one not caused by the death of a monarch.

Wearing a magenta outfit, Margrethe signed her abdication during a meeting with the Danish Cabinet at the Christiansborg Palace, a vast complex in Copenhagen that houses the Royal Reception Rooms and Royal Stables as well as the Danish Parliament, the prime minister’s office and the Supreme Court.

Prime Minister Mette Frederiksen next proclaimed Frederik King from the balcony of the palace in front of thousands of people.

Ms Frederiksen read the proclamation three times, which is the tradition, as Frederik stood beside her wearing a ceremonial military uniform adorned with medals.

He was then joined on the balcony by the new, Australian-born Queen Mary and the couple’s four children, and the crowd spontaneously sang the national anthem.

“My hope is to become a unifying king of tomorrow,” Frederik said. “It is a task I have approached all my life.”

It is the custom for each new sovereign to adopt a royal motto as a guiding principle for their reign, and Frederik’s is: “United, committed, for the kingdom of Denmark.”

“I want to return the trust I meet,” the new king said. “I need trust from my beloved wife, you and that which is greater than us.”

Frederik then kissed Mary, who wore a white dress, and another great cheer rose from the crowd.

They then left Christianborg Palace in a horse-drawn coach as church bells rang out, and headed to their Amalienborg residence, where they once again appeared before a huge crowd of people cheering and waving the nation’s flag of a white cross on a red background.

Frederik, who was visibly moved, placed both hands on his heart in a gesture of thanks.

The abdication document had been presented to Margrethe as she sat at a massive table covered in red cloth around which royals and members of the Danish government were seated. Frederik, who was in a ceremonial military uniform adorned with medals, sat beside her.

After signing it, Margrethe rose and gestured to Frederik to take her place. “God save the king,” she said as she left the room.

The abdication leaves Denmark with two queens: Margrethe keeps her title while Frederik’s wife becomes Queen Mary. Frederik and Mary’s eldest son, Christian, 18, has become Crown Prince and heir to the throne.

Christian handed Margrethe her walking stick as she left the abdication ceremony.

Citing health issues, Margrethe announced on New Year’s Eve that she would step down, stunning a nation that had expected her to live out her days on the throne, as is tradition in the Danish monarchy. She underwent major back surgery last February and did not return to work until April.

Even the prime minister was unaware of her intentions until just before the announcement. Margrethe had informed Frederik and his younger brother Joachim just three days earlier, the Berlingske newspaper wrote, citing the royal palace.

People from across Denmark gathered outside parliament, with many swarming streets decorated with the red and white Danish flags.

Several shops had hung up photos of the queen and king-to-be, while city buses were adorned with smaller Danish flags as is customary during royal events.

Many others across the kingdom of nearly six million people followed a live television broadcast of the historic event.

The royal guards’ music band made their daily parade through central Copenhagen but wore red jackets, instead of their usual black, to mark major events.

Copenhagen resident Rene Jensen, wearing a replica of a royal robe and a bejewelled purple crown on his head, said he expects Frederik to be “a king for the nation, representing us everywhere”.

The last time a Danish monarch voluntarily resigned was in 1146 when King Erik III Lam stepped down to enter a monastery. Margrethe abdicated on the same date she ascended the throne following the death of her father, King Frederik IX.

Denmark’s monarchy traces its origins to 10th-century Viking king Gorm the Old, making it the oldest in Europe and one of the oldest in the world. Today the royal family’s duties are largely ceremonial.

Australians also turned out on the streets of Copenhagen to celebrate one of their own becoming queen.

“I think it’s good that she’s not from royalty and has a normal Australian background. We can relate more to that, because she’s from a middle-class background, and we are too,” said Judy Langtree, who made the long journey from Brisbane with her daughter to witness the royal event.

A survey commissioned by Denmark’s public broadcaster DR published on Friday showed that 79% of the 1,037 people polled by the Epinion polling institute said they believed Frederik is ready to take the reins and 83% said they think his wife Mary is ready to become queen. The survey margin of error was three percentage points, DR said.