(AFP) - Denmark's King Frederik X acceded to the throne on Sunday, ushering in a new era after his mother Queen Margrethe abdicated, with more than 100,000 Danes turning out for the unprecedented event.

After a final horse-drawn carriage procession through the streets of Copenhagen, the hugely popular 83-year-old queen signed a declaration of abdication at Christiansborg Palace, ending her 52-year reign and automatically making her son monarch.

She then left the Council of State, also attended by the government, the new king, his wife and their 18-year-old eldest son, the new Crown Prince Christian.

Margrethe left the room with tears in her eyes, saying: "God bless the king."

In front of a sea of Danes braving the winter chill and waving red-and-white flags, Prime Minister Mette Frederiksen then proclaimed 55-year-old Frederik the new king on the balcony of Christiansborg Palace, the seat of parliament and government.

Frederik, clad in his gala admiral's uniform with gold epaulettes and blue sash, blinked back tears and smiled as he waved to the crowd, estimated at more than 100,000 people by media outlets.

Like his mother, Frederik is hugely popular, enjoying the support of more than 80 percent of Danes.

"My mother succeeded like few others in becoming one with her country," he told the crowd.

"My hope is to become a unifying king for the future... It's a responsibility I assume with respect, pride and a lot of happiness," he said, repeatedly placing his hand on his heart as the crowd cheered.

"I think he's going to be a great king. I'm looking forward to seeing how he's going to do it and see him show more of himself," Rene Jensen, a Copenhagen resident fitted out for the day in a red velvet robe and crown, told AFP.

Frederik was then joined on the balcony by his Australian-born wife Mary, wearing a white dress with her dark hair swept back in a bun, and their four teenaged children.

Mary is the first commoner to become queen in Denmark.

The crowd erupted in loud cheers as the new king and queen kissed on the balcony.

The royal standard was then raised at the couple's residence at the Amalienborg Palace.

- 'Soul of the nation' -

In line with Danish tradition, no foreign dignitaries or royals were invited on Sunday, and there was no coronation or throne.

Britain's King Charles congratulated Frederik and Mary on their accession in a message posted on the Danish court's Instagram account, while Sweden's King Carl XVI Gustaf thanked Queen Margrethe, whom he referred to as "cousin Daisy", for their "good cooperation" over the years.

US President Joe Biden paid tribute to his Queen Margrethe's "incredible example of selfless, principled service".

This is only the second time a Danish sovereign has stepped down -- the last was Erik III, almost nine centuries ago in 1146.

Portraits and banners around the capital thanked the queen for her years of service, with cheeky signs in the metro declaring "Thanks for the Ride, Margrethe".

Others read "Long Live the King".

Margrethe chose to abdicate exactly 52 years to the day after she took over from her father, Frederik IX.

The queen stunned Danes when she announced her abdication in her annual televised New Year's Eve address, after having repeatedly insisted she would follow tradition and reign until her death.

Even her own family was only informed three days prior.

She attributed her decision to health issues after undergoing major back surgery last year.

Aske Julius, a 27-year-old Copenhagen resident, called Margrethe "the embodiment of Denmark... the soul of the nation."

"More than half of the Danish population has never known anything else but the queen," he said.

Margrethe will retain her title of queen.

- 'Different' monarch -

Experts say her decision to pass the baton now will give Frederik time to flourish in his role as monarch.

He is expected to bring his own, more informal style to the monarchy, which dates back to the 10th-century Viking era.

"He feels like one of us ... He tries to be a lot like normal people," onlooker Berit Nissen told AFP on Sunday.

"They raised their children like normal families do," she said, referring to the couple's decision to send their kids mainly to state schools.

While his mother is known for her love of the arts and is an accomplished writer and artist, Frederik is an avid sportsman who champions environmental causes.

In Denmark, the monarch's role is largely ceremonial, but the sovereign does sign legislation, formally presides over the forming of a government and meets with the cabinet regularly.

© Agence France-Presse