In the speech, the new head of state revealed that he planned to serve for life, meaning that William will only succeed him when he passes away. Usually the monarch serves for life, although there have been a few cases of abdications, most recently with King Edward VIII, who abdicated the throne in order to marry twice-divorced American heiress Wallis Simpson.
WATCH: A new monarch: The life and times of King Charles III
In his speech, Charles said: "As the Queen herself did with such unswerving devotion, I too now solemnly pledge myself, throughout the remaining time God grants me, to uphold the Constitutional principles at the heart of our nation."
It marked an incredibly emotional day for the royal who headed back from Balmoral earlier on Friday, following the death of his mother, the Queen.
Charles was by Her Majesty's side on Thursday when she passed away, alongside his sister Princess Anne. Other members of the family, including Prince William, Prince Andrew, Prince Edward, Sophie Wessex and Prince Harry arrived after the Queen had passed.
King Charles released a statement paying tribute to his mother, which read: "The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family. We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother.
"I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world. During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which The Queen was so widely held."
At the time, Charles left the statement unsigned as a mark of respect for his mother, who reigned for 70 years.
Charles viewed floral tributes left for his mother earlier in the day
Charles will be formally proclaimed monarch at an Accession Council on Saturday. While the King has automatically received the title following the death of his mother, the Accession Council takes place within 24 hours of the death of a sovereign, usually at St James's Palace in London.
In this case, the time has been extended as Her Majesty's death was announced on Thursday evening.
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