The former Prince of Wales ascended to the throne following the death of his mother, Queen Elizabeth II, on Thursday.
He was then formally proclaimed King at a historic ceremony in St James’s Palace following a meeting of the accession council during which Charles swore an oath to privy counsellors.
Proclamations will take place in other parts of the UK, including Wales, at about midday today.
Up to 2,000 people will be allowed to attend the event, with spaces inside the grounds available on a first come, first served basis. Gates are expected to open at 10am.
Several main roads through the city centre are due to be closed between 7am and 2pm.
Prior to the Proclamation, 26 men of the 3rd Battalion the Royal Welsh – supported by the Band of the Royal Welsh and accompanied by the regimental mascot, a Welsh billy goat called Shenkin – will march from City Hall at 11.25am along Boulevard de Nantes, North Road and Duke Street to Cardiff Castle.
At the castle, the Wales Herald of Arms Extraordinary, Tom Lloyd, will make the proclamation in English and the Lord Lieutenant of South Glamorgan, Morfudd Meredith, will proclaim King Charles in Welsh.
After the readings, members of 104th Regiment of the Royal Artillery will fire a 21-gun salute before the singing of God Save The King and Hen Wlad Fy Nhadau, Wales’ national anthem.
It will be the third time in three days that canon fire has resounded across the city to mark both the Queen’s death and her son Charles’s accession to the throne.
Flags on the castle and council buildings, which were at half-mast were returned to full-mast on Saturday, to coincide with the Reading of the Principal Proclamation of the new monarch in London.
Flags will return to half-mast at 1pm on Sunday after the Proclamation is read in Cardiff.
The Senedd has also been recalled on Sunday at 3pm to allow members to pay tribute to the Queen.
However, all other business has been suspended until after the state funeral.