Denmark’s Queen Margrethe II signed her historic abdication on Sunday, a step which meant her son immediately became King Frederik X.

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

Many 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 in 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.

The document was presented to her as she sat at a massive table covered in red cloth around which royals and members of the Danish government were seated.

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

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

Prime Minister Mette Frederiksen will next proclaim Frederik king from the balcony of the palace before thousands of people.

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.

Margrethe underwent major back surgery last February and did not return to work until April.

Even the prime minister was unaware of the Queen’s 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 day 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.

Unlike in the UK, there is no coronation ceremony in Denmark. The prime minister will formally proclaim Frederik king from the balcony of Christiansborg Palace.

Four guns at Copenhagen harbour will fire a salute to mark the succession.

Later in the day, Copenhagen’s Tivoli Gardens plans to celebrate the new King and Queen with the biggest fireworks display in the amusement park’s 180-year history.

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 3 percentage points, DR said.