At the stroke of noon (2000 AEST) on Saturday, the crown prince - flanked by his wife, Crown Princess Mary, and their four children, and Frederik's mother, Queen Margrethe - stepped out on a balcony at the Frederik VIII's Palace and waved to the crowds.

The queen then led the crowds in ten cheers for Frederik, who smiled broadly.

The royals then stepped back inside. But, to the delight of the crowds at the Amalienborg Palace Square, stepped out again two more times.

People sang birthday songs. Many waved red-and-white flags.

Frederik and his Australian-born wife were to ride in a horse-drawn carriage to Christiansborg Palace to attend a gala banquet hosted by the queen in the evening. Guests expected include members of the Danish royal family and guests from Denmark and abroad.

Foreign royals due at the banquet include the king and queen of Sweden, Swedish Crown Princess Victoria and her husband, and the Norwegian crown prince couple.

A survey published on Saturday showed that eight out of 10 Danes believe that Frederik will be a "very good" or "mainly good" king, a slight increase compared to 2016. The survey was commissioned by broadcaster DR.

Frederik is known for his interest in sports and training. He has cited his marriage with Mary as a key turning point in his life.

Several events have been staged to mark Frederik's birthday, including a Royal Run in five cities earlier this week.

Australian Associated Press