In Edinburgh, the King’s Body Guard for Scotland, known as the Royal Company of Archers, and the guard of honour marched from the Castle Esplanade to Mercat Cross for the ceremony.
They were joined by soldiers from the Royal Regiment of Scotland, where a drill took in front of the cross in Edinburgh.
The Lord Lyon King of Arms read out the proclamation before the national anthem was sung. Three cheers were then carried out for the new monarch.
At the event at Hillsborough Castle, in Northern Ireland, a Proclamation Guard comprising one officer and 20 other ranks was provided by The 2nd Battalion The Royal Irish Regiment together with the band of the Royal Irish Regiment.
Prior to the proclamation being read out, a 40-second fanfare was sounded by a bugler.
A bell in the clock tower of the Court House at Hillsborough Castle gave a single chime on the stroke of 12 noon to signal the start of the proclamation.
Following the reading of the proclamation, a 21-gun salute was fired by 206 (Ulster) Battery Royal Artillery.
In Wales, the Herald of Arms Extraordinary, Tom Lloyd, and the Lord-Lieutenant of South Glamorgan, Morfudd Meredith, have read the proclamation of the new King in English and Welsh to the people of Wales at Cardiff Castle.
After the readings, members of 104th Regiment of the Royal Artillery fired a 21-gun salute before the singing of God Save The King and Hen Wlad Fy Nhadau, Wales's national anthem.
It was the third time in three days that artillery fire has resounded across the Welsh capital to mark both the Queen's death and the accession of her son to the throne.
Welsh Secretary Robert Buckland is attended the Proclamation ceremony, along with Wales First Minister Mark Drakeford and opposition leaders.