Huge crowds gathered in Copenhagen on Sunday as Queen Margrethe abdicated after 52 years on the throne, in favour of her son who became King Frederik X.

Thousands of people began assembling early Sunday outside Copenhagen's Christiansborg Palace despite the winter chill, many bundled up in warm bonnets and ski pants to ward off the freezing temperatures.

"We're very excited to see this historic event ... it's going to be very big, a very important moment in Danish history," said 35-year-old Renee Jense, decked out with her friends in playful red velvet robes and crowns.

The hugely popular Queen Margrethe II, 83, left her residence at Copenhagen's Amalienborg Palace shortly after 1:30 pm for a short carriage ride to Christiansborg Palace, the seat of government and parliament.

There, at a Council of State at 2 pm, she signed a declaration of abdication ending her 52-year reign, only the second time a Danish sovereign has stepped down, the last one Erik III almost nine centuries ago in 1146.

Her 55-year-old son Frederik -- who was in the Council of State along with his Australian-born wife Mary and their eldest child, 18-year-old Prince Christian -- automatically became king and head of state upon Margrethe's abdication.

Prime Minister Mette Frederiksen proclaimed him King Frederik X on the balcony of Christiansborg Palace.

There was a heavy police presence in the capital, decked out in red-and-white flags for the occasion. Copenhagen police official Peter Dahl told AFP he expected "more than 100,000 people" in the streets.

- 'Soul of the nation' -

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. 

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".

Queen Margrethe II of Denmark (C) rides in a gold carriage as she is escorted by the Guard Hussar Regiment's Mounted Squadron from Amalienborg Castle to Christiansborg Castle for her proclamation of abdication in Copenhagen, Denmark.

Apart from the abdication, the protocol is largely similar to previous royal successions in Denmark.

No foreign dignitaries or royals are invited, and there is no coronation or throne for the new monarch.

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

"There's a lot of symbolism around this day," Cecilie Nielsen, royal correspondent for Danish public broadcaster DR, told AFP.

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.

Opinion polls show that more than 80 percent of Danes support her decision.

Margrethe will retain her title of queen and may represent the royal family on occasion. 

- 'Different' monarch -

Experts say that passing the baton to her son now will give him time to flourish in his role as monarch, after gradually taking on increasing responsibilities. 

"She thinks the crown prince is totally ready to take over. And she wants to avoid a situation like in Great Britain where Prince Charles became King Charles after the age of 70," historian Hovbakke Sorensen said.

Like his mother, Frederik, who has been crown prince since the age of three, enjoys the support of more than 80 percent of Danes.

But he is expected to bring his own style to the monarchy, which dates back to the 10th century Viking era.

"Queen Margrethe II is a woman of her time and Frederik also lives in his own era. He understood that he could not copy her and has managed to define his own image, his own ties to the Danish people," another historian, Bo Lidegaard, told AFP. 

"We will have a different type of monarch, much more informal in his way of speaking with people when he travels across the country," his colleague Hovbakke Sorensen added.

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 he or she does sign legislation, formally presides over the forming of a government and meets with the cabinet regularly.