Thousands of people have gathered in Copenhagen to witness a royal succession in one of the world’s oldest monarchies.

Queen Margrethe II will sign her abdication at a meeting with the Danish Cabinet following more than five decades of service. About an hour later, her 55-year-old son and crown prince Frederik will be proclaimed king before the people on the balcony of Christiansborg Palace in the heart of the Danish capital.

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

As the moment neared, Margrethe left her residence for the Christiansborg Palace in a horse-drawn coach escorted by members of the Hussar Regiment in blue uniforms with red jackets on horses. There she is set to formally sign her abdication.

Her son and his wife, Crown Princess Mary, also left for the palace in a limousine. Later the royal standard will be lowered on Margrethe’s home and raised on the building where Frederik and Mary live.

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 Prime Minister Mette Frederiksen was unaware of the queen’s intentions until right before the announcement. Margrethe had informed Frederik and his younger brother Joachim just three days earlier, the Berlingske newspaper wrote, citing the royal palace.

The abdication will leave 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 will be abdicating 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 that they believed Frederik was prepared to take the reigns and 83% said they thought his wife Mary was ready to become queen. The survey margin of error was 3 percentage points, DR said.