Thousands of people will gather Sunday in downtown Copenhagen to witness a historic moment in one of the world’s oldest monarchies.

Around 2 p.m. (1300 GMT), Queen Margrethe II will sign her abdication at a meeting with the Danish Cabinet following over five decades of service. About an hour later, her 55-year-old son and crown prince will be proclaimed King Frederik X 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.

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 didn’t 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 queen has been here and she has been on the throne for more than 50 years. And she is an extremely respected and popular figure, so people hadn’t expected this and they were in shock,” Danish royal expert Thomas Larsen said.

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.

People began gathering early Sunday outside parliament, with scores already swarming the streets of Copenhagen’s main shopping street, the Pedestrian Stroeget which was decorated with the red-and-white Danish flags. Several shops hung photos of the Queen and king-to-be. Local media said hundreds were heading to the Danish capital from across the country and others were following the live TV broadcast to witness the monumental event.

City buses were adorned with smaller replicas of the Danish flag as is customary during royal events.

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. Thousands of Danes are expected to witness the proclamation from the square below.