The land of Hans Christian Andersen fairy tales has inked a modern-day classic with Frederik and his Australian-born wife Mary sharing a kiss after becoming the new King and Queen of Denmark.
Beloved chain-smoking octogenarian monarch Margrethe II, who has been on the throne for 52 years, flagged in a bombshell New Year's Eve message, that she would be the first Danish royal to abdicate the throne in 900 years.
The hugely popular royal couple are set to bring generational change to the Danish throne, more than 23 years after their fairy tale romance started in a Sydney pub during the 2000 Olympics.
Tens of thousands of people braved Copenhagen's 3degC temperature to catch a glimpse of the new monarchs as church bells rang out across the capital.
Mary, 51, was dressed in a white gown while Frederik wore full gala uniform and a blue sash.
The Danish royal transition was a low-key affair.
Frederik became Denmark's head of state, Mary the Queen and their eldest son 18-year-old Christian the Crown Prince, the moment Margrethe signed abdication papers at a state council meeting inside Christiansborg Palace, home to the Danish parliament.
Danish Prime Minister Mette Frederiksen officially proclaimed King Frederik X - a former party boy prince turned family man - on a balcony at the Danish parliament overlooking the flag-waving masses, as confetti rained down over the crowd.
''My hope is to be a unifying King of tomorrow,'' he told the crowd.
''It's an assignment I've been preparing for my entire life. It's a responsibility I'm taking on with respect, pride and immense joy.''
"I need all the support I can get from my beloved wife, my family and from God."
Queen Mary joined her husband on the balcony and the crowd went wild when they shared a kiss.
Their children Prince Christian, Princess Isabella, 16, and twins Prince Vincent, 13, and Princess Josephine, 13, also made an appearance.
The crowd broke out into the Danish national anthem.
Among the sea of red and white Danish flags were some blue ensigns decked with the Southern Cross - a nod to Mary's homeland.
Brisbane biomedical science university graduate Ebony Wilson, 25, and her grandmother Judy bought a last-minute plane ticket for Copenhagen on Wednesday to be part of the moment in history.
"I was thinking about it and then I thought, why not be spontaneous? It's a once-in-a-lifetime opportunity,'' Ms Wilson told AAP in Copenhagen.
There were more celebratory explosions earmarked at Copenhagen's famed Tivoli Gardens amusement park - the biggest fireworks extravaganza in its 180-year history.
Hundreds of Aussie expats on the Australian embassy's email database affectionately known as the "Vegemite List" joined ambassador Kerin Ayyalaraju for a soiree to toast Mary's promotion.
"The Queen of Denmark understands Australia like no other queen does," she told the crowd, standing next to the embassy's giant koala mascot.
"She'll know what we mean if we talk about sitting on a beach eating a melting Cherry Ripe, watching Neighbours and the Australian summer combination of sunburn, mozzie bites and magpies warbling. We know she gets us."
In Australia, royal enthusiasts celebrated at the Sydney's Slip Inn where the two first met, and in Mary's home state of Tasmania, where landmarks were lit up in Denmark's red and white colours.
The royal couple will kick off their new roles on Monday with a reception at the Danish parliament.
