A Royal Navy officer fainted just moments before taking part in the procession heading back to Buckingham Palace for the King's Coronation.
A small number of troops rushed to assist their colleague who was quickly put onto a stretcher and given medical assistance.
More than 4,000 ceremonial troops took part in the procession, which began at 1.07pm, to escort the King and Queen back to Buckingham Palace in the Gold State Coach.
The King was crowned the nation's monarch and prayed to be a "blessing" to people of "every faith and conviction".
Charles became the 40th reigning sovereign to be crowned at Westminster Abbey, the nation's coronation church since 1066, as Archbishop of Canterbury Justin Welby placed St Edward's Crown on his head.
The historic moment, watched around the globe, was a fulfilment of the King's destiny, but followed the death of his mother, Queen Elizabeth II, last September after a 70-year reign.
In a touching moment, the Prince of Wales pledged himself to his father the King, kissing him on the cheek and touching St Edward's Crown.
As William knelt before Charles, who held his son's hand between his palms, the future monarch said: "I, William, Prince of Wales, pledge my loyalty to you and faith and truth I will bear unto you, as your liege man of life and limb. So help me God."
The King's younger son the Duke of Sussex was among the congregation, sitting two rows behind his brother with the Duke of York's family, and he was seen intently watching the crowning.
And when a few minutes later the congregation was invited to pay homage to the new monarch, Harry was seen, along with the other royals around him, speaking the words: "God save King Charles. Long live King Charles. May the King live forever."
The Coronation was a deeply religious ceremony steeped in symbolism and its prayers took the theme of "Called to Serve", an attribute associated with the late Queen who pledged her life to the Commonwealth.
Before the crowning the archbishop delivered a sermon to the 2,300 guests, a gathering of world leaders, celebrities, UK politicians, foreign royalty, everyday heroes and the royal family.
Mr Welby began by telling the congregation: "We are here to crown a King, and we crown a King to serve.
"What is given today is for the gain of all. For Jesus Christ announced a kingdom in which the poor and oppressed are freed from chains of injustice. The blind see. The bruised and broken-hearted are healed."
Speaking to Charles and Camilla, Mr Welby said: "The weight of the task given you today, Your Majesties, is only bearable by the spirit of God, who gives us the strength to give our lives to others.
"With the anointing of the Holy Spirit, the King is given freely what no ruler can ever attain through will, or politics, or war, or tyranny: the Holy Spirit draws us to love in action."
Charles delivered a King's Prayer, the first time a monarch has spoken words to God aloud during a coronation, and he touched on the duty of the sovereign to serve all communities.
He told the abbey: "God of compassion and mercy whose son was sent not to be served but to serve, give grace that I may find in thy service perfect freedom and in that freedom knowledge of thy truth.
"Grant that I may be a blessing to all thy children, of every faith and conviction, that together we may discover the ways of gentleness and be led into the paths of peace. Through Jesus Christ our Lord, amen."
