Australian-born Princess Mary becomes Queen of Denmark

Prince Frederik X and Australian-born Princess Mary have been proclaimed King and Queen of Denmark on the balcony of Christiansborg Palace in Copenhagen.

Danish Prime Minister Mette Frederiksen declared their ascension to the throne before thousands of people on Sunday afternoon (local time), making Mary the first Australian queen to reign over a European country.

The succession came two weeks after Queen Margrethe II — who in the past had said she would remain on the throne for life — stunned the country on New Year's Eve when she announced her decision to abdicate after 52 years of service.

Although she did not give an exact reason for her decision to step down, she said that a major back surgery she underwent last February had made her think about her future.

On Sunday, Ms Frederiksen read the proclamation three times, which is the tradition, as Frederik, 55, stood beside her wearing a ceremonial military uniform.

He was then joined on the balcony by Queen Mary, 51, and the couple's four children, as the crowd spontaneously sang the national anthem.

"My hope is to become a unifying king of tomorrow," Frederik said in his first speech as King.

"It is a task I have approached all my life. It is a task I take on with pride, respect and joy.

​​Frederik married Tasmanian law graduate and former real estate agent, Mary Elizabeth Donaldson, in 2004 after a chance encounter at a Sydney pub four years earlier.

The royal couple will continue to reside with Margrethe, who will retain her title as Queen, in Amalienborg, albeit in their respective palaces in the octagonal complex.

An hour before the proclamation, Margrethe formally stepped down from the throne in a muted ceremony.

The 83-year-old queen signed the official declaration of her abdication during a Council of State meeting at the Christiansborg Palace.

It was attended by government representatives, Frederik and Mary, as well as the couple's oldest son Christian, 18, who is the new heir to the throne.

This is an historic moment for the nation of nearly 6 million people, with Margrethe the first Danish monarch in nearly 900 years to voluntarily relinquish the throne.

But unlike the lavish ceremonies held for the English monarchy, there were no formal coronations or anointments.

After signing her abdication papers, Margrethe rose and gestured to Frederik to take her place. "God save the king," she said as she left the room using a cane for support.

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 of January that she ascended the throne following the death of her father, King Frederik IX, on January 14, 1972.

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.

The new king and queen take the throne at a time of huge public support and enthusiasm for the monarchy.

The most recent survey done after Margrethe announced she would abdicate indicated that 82 per cent of Danes expect Frederik to do well or very well in his new role, while 86 per cent said the same about Mary.

Despite close to freezing temperatures on Sunday, thousands of people from all over Denmark gathered outside Christiansborg Palace to witness the succession.

The palace houses the Danish Parliament, the prime minister's office and the Supreme Court.

Copenhagen's main shopping street, the Pedestrian Stroeget, was decorated with red-and-white Danish flags, while several shops hung photos of Margrethe and the king-to-be.

Australian flags were also seen among the crowds, with some Australians joining the festivities in 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," Judy Langtree, who made the long journey from Brisbane to witness the event, said.

"We have come here today because this is history being made in front of our eyes. We just had to be here," Soren Kristian Bisgaard, a 30-year-old pilot, said.

He was drinking champagne with three friends, sitting in camping chairs in front of Christiansborg.

"I'm very fond of the royal family. I have been in the Royal Life Guards myself, standing guard at the royal palace. I'm very proud to have done that and also to be here today," Mr Bisgaard said.

Royals across Europe sent their congratulations, including Britain's King Charles III, whose late mother Queen Elizabeth II and Margrethe were third cousins.

Charles said he was committed to working with them "on ensuring that the enduring bond between our countries, and our families, remains strong". (ABC)