The Prime Minister, politicians, dignitaries and members of the public gathered in St Paul's Cathedral on Friday evening for a service of prayer and reflection after the passing of the Queen on Thursday.
Some 2,000 people attended the service led by the Archbishop of Canterbury, during which the first official rendition of "God Save the King" was sung.
King Charles III's first address to the nation as sovereign was broadcast live to the service in which he paid tribute to his "darling mama" and talked about his "deeply rooted" Christian faith.
"The role and the duties of Monarchy also remain, as does the Sovereign's particular relationship and responsibility towards the Church of England - the Church in which my own faith is so deeply rooted," the King said.
"In that faith, and the values it inspires, I have been brought up to cherish a sense of duty to others, and to hold in the greatest respect the precious traditions, freedoms and responsibilities of our unique history and our system of parliamentary government.
"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."
Addressing the congregation, the Bishop of London, Sarah Mullally, said the Queen had lived an "extraordinary" life that was "dedicated to others".
"A life lived in the service of others is a rare jewel. It is a jewel that her late Majesty the Queen wore as a crown," she said.
While the UK and the world saw remarkable change during her 70-year reign, the Queen had remained a "remarkable constant in the lives of millions, a symbol of unity, strength, forbearance, and resilience," the bishop continued.
"She has been this nation's unerring heartbeat through times of progress, joy and celebration as well as in much darker and more difficult seasons," she said.
Prime Minister Liz Truss read from Romans 14:7-12, which speaks about the account that all will be called to give before God and contains the reassurance for Christians of belonging to the Lord even in death:
She said: "We do not live to ourselves, and we do not die to ourselves. If we live, we live to the Lord, and if we die, we die to the Lord; so then, whether we live or whether we die, we are the Lord's.
"For to this end Christ died and lived again, so that he might be Lord of both the dead and the living.
"Why do you pass judgement on your brother or sister? Or you, why do you despise your brother or sister? For we will all stand before the judgement seat of God.
"For it is written, 'As I live, says the Lord, every knee shall bow to me, and every tongue shall give praise to God.' So then, each of us will be accountable to God."
Others in attendance were leader of the opposition, Sir Keir Starmer, London Mayor Sadiq Khan, Chancellor Kwasi Kwarteng, Foreign Secretary James Cleverly, Deputy Prime Minister Therese Coffey, Leader of the Commons Penny Mordaunt and Welsh Secretary Robert Buckland.
Hymns sung during the service included 'O Thou Who Camest From Above'. A piper played 'Flowers of the Forest', the same lament that was played at the funeral of the Queen's late husband, Prince Philip, Duke of Edinburgh, last year.
Tickets for the service were given to the public on Friday on a first-come-first-served basis, with the cathedral closed throughout the day to other visitors. It was to reopen on Saturday for private prayer and services.
Earlier in the day, bells tolled for an hour at midday at the cathedral as well as Westminster Abbey and other churches and cathedrals across the country in honour of the Queen.