St Edward's Crown is placed on top of the King's head by the Archbishop of Canterbury - and he is officially crowned.
The congregation cries "God save the King", the abbey bells ring and a fanfare sounds.
Gun salutes are also fired across the UK to mark the moment.
Some 5,000 members of the armed forces are arriving by train at London Waterloo station.
The ceremony itself will be led by the Archbishop of Canterbury at 11:00 BST (10:00 GMT) and attended by the Royal Family as well as world leaders and dignitaries.
Thousands of people turned central London red, white and blue on Saturday, with a sea of Union Jack flags lining the streets for the coronation of King Charles III. While the coronation service itself promises to be a sombre and sober affair, a celebratory atmosphere was building in the British capital and beyond. On The Mall outside Buckingham Palace, the most fervent royal fanatics have already got the party started, with some camping out for days to secure a prime spot for the historic event.
Westminster Bridge. Thousands of tri-service personnel crossing the Thames after arriving at Waterloo Station. Heading to different assembly points to fulfill their roles in the #Coronation pic.twitter.com/FHSep6nMTG— Tom Sables (@TomSables) May 6, 2023
-Vice President Jagdeep Dhankhar
-Actor Sonam Kapoor
-Two dabbawalas from Mumbai
-Pune-born architect Sourabh Phadke
-Prince’s Trust Global Award winner Gulafsha
-Indian-origin Jay Patel
Both are expected to perform at the coronation concert in Windsor tomorrow.
"This is not an everyday occurrence, I'm walking into history. This is the greatest show on earth right now. To have an invite to this is everything", said Lionel Richie.
The Coronation Vestments, comprising of the Supertunica (L) and the Imperial Mantle (R) are displayed in the Throne Room at Buckingham Palace in London on April 26, 2023.
VIDEO: British troops march through London ahead of King Charles III's coronation at Westminster Abbey. pic.twitter.com/hFWW3cvjjB— AFP News Agency (@AFP) May 6, 2023
King Charles & Queen Camilla travelled to Westminster Abbey in the Diamond Jubilee State Coach made for the Queen’s 80th birthday https://t.co/YjjauTSgzJ— Alison Eastwood (@AlisonEastwood) May 6, 2023
Canada's Prime Minister Justin Trudeau and his wife Sophie Trudeau walk outside Westminster Abbey ahead of Britain's King Charles' coronation ceremony, in London, Britain 📷 Lisi Niesner pic.twitter.com/IXJV36mfqv— Reuters (@Reuters) May 6, 2023
The King’s procession has reached Westminster Abbey while the Prince and Princess of Wales are on their way. The service is set to begin in about ten minutes and is expected to last two hours, shorter than many coronations in history, the Guardian reports.
Prince George walked behind King Charles III as a Page of Honour during the coronation ceremony and became the youngest future monarch to take part in such an event.
The ceremony is expected to last for at least two hours.
This is the moment at which the King is presented to the congregation at each of the cardinal points of the compass in turn.
Spectacular scenes in London on #Coronation Day.— The Royal Family (@RoyalFamily) May 6, 2023
Travelling in the Diamond Jubilee State Coach, accompanied by The Sovereign’s Escort of Household Cavalry, The King and The Queen have arrived at Westminster Abbey for the Coronation Service. pic.twitter.com/BDkklcj9pA
The moderator of the General Assembly presents the Bible to the King, on which he swears the Coronation Oath.
"Are you willing to take the oath?" the Archbishop of Canterbury asks.
"I am willing," the King replies.
He then swears the Accession Declaration Oath.
"I, Charles, do solemnly and sincerely, in the presence of God, do profess, testify and declare that I am a faithful Protestant," he says.
The former is taken at every coronation, while the latter is taken at whichever of the coronation or a state opening of parliament comes first.
He signs both oaths.
Rishi Sunak reads the Epistle, continuing the tradition of British prime ministers reading at state occasions.
Mr Sunak is a practising Hindu reading from a biblical book, and it's worth noting that the King has encouraged a multi-faith theme for the ceremony.
The passage - Colossians 1:9-17 - is said to have been chosen to reflect the theme of service to others.
Here at Westminster Abbey for the #Coronation of King Charles III pic.twitter.com/hF704nBtsK— The Prince and Princess of Wales (@KensingtonRoyal) May 6, 2023
Congratulations to King Charles III and Queen Camilla on their Coronation. The enduring friendship between the U.S. and the U.K. is a source of strength for both our peoples.— President Biden (@POTUS) May 6, 2023
I am proud the First Lady is representing the United States for this historic occasion.