Denmark’s Queen Margrethe II signed her historic abdication on Sunday, paving the way for her son, Frederik X, to immediately become king, Danish broadcaster DR reported.

Margrethe, 83, is the first Danish monarch to voluntarily relinquish the throne in nearly 900 years since King Erik III Lam stepped down to enter a monastery in 1146.

She 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.

Frederik, 55, was present in the room.

Prime Minister Mette Frederiksen will proclaim him king on the balcony of the palace in front of a crowd of thousands of people.

Frederik’s 18-year-old son, Christian, who becomes Denmark’s crown prince and heir to the throne, was also in attendance.

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 Ms Frederiksen was unaware of the queen’s intentions until just before the announcement.

Margrethe had informed Frederik and his younger brother Joachim only three days earlier, the Berlingske newspaper wrote, citing the royal palace.

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.

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

Several shops hung 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 TV broadcast of the historic event.

The royal guards’ music band made their daily parade through 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, which houses government offices, Parliament and the Supreme Court as well as the Royal Stables and Royal Reception Rooms.

Four guns on the Copenhagen harbour will fire three times 27 rounds to mark the succession. In the late afternoon, Copenhagen’s Tivoli Gardens amusement park plans to celebrate the new king and queen with the biggest fireworks show in the 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.