The coronation of King Charles III and Queen Camilla

King Charles and Queen Camilla have entered the Abbey to the strains of “I was glad,” the stirring coronation anthem written by Hubert Parry for the coronation of King Edward VII in 1902 and sung at all coronations since then.

The choral music for today’s ceremony is performed by the Choir of Westminster Abbey, augmented by the Choir of His Majesty’s Chapel Royal, St. James’ Palace; choristers from Methodist College, Belfast; the Choir of Truro Cathedral and an octet from the Monteverdi Choir.

Parry incorporated a set of “vivats” – traditional Latin acclamations for the new monarch – into his piece, which must be rewritten for each coronation.

Today we hear the King’s Scholars of the prestigious Westminster School in London, singing Vivat Regina Camilla! Vivat Rex Carolus! (Long live Queen Camilla! Long live King Charles!).

The King and Queen have arrived at Westminster Abbey, with the start of the historic coronation ceremony just moments away.

After some will-they-won't-they speculation, Prince Harry accepted the invitation to his father's coronation, but he's without his wife, the Duchess of Sussex.

Meghan is back in California with the couple's two children as today is also Prince Archie's fourth birthday.

There's been heavy speculation in the lead-up to today's celebrations over how much involvement Harry will have and whether his presence means the family is moving on from the rift that saw the duke and duchess step back from their role as senior members of the royal family.

The palace previously said it wouldn't comment on whether the duke will appear on Buckingham Palace's famous balcony following the ceremony at Westminster Abbey. In fact, the palace is not confirming who will join the King on the balcony ahead of time at all. So that'll be a moment we'll have to wait for later.

CNN royal historian Kate Williams previously described Harry's appearance today as a "flying visit."

"[Prince Harry] is coming for this major event of his father's coronation but it's not going to be a family reunion. We're not going to see lots of big family meet-ups. Certainly, there has been damage done," she explains.

Williams adds that his presence is a "show of unity" but that we'll need to see what happens at the service.

As King Charles nears Westminster Abbey, several senior members of the royal family have entered the venue.

His siblings -- Princess Anne, Prince Edward and Prince Andrew -- have all taken their seats.

Earlier Queen Camilla’s family arrived at the Abbey, including her former husband Andrew Parker-Bowles and her daughter Laura Lopes and son Tom Parker-Bowles.

Prince Harry has arrived at the ceremony, among the first group of royals to enter the Abbey.

Harry is attending without his wife, Meghan, and their children, who have remained in the US.

The procession, which is underway through the streets of central London, should take 33 minutes.

A total of 200 personnel and horses are involved in the King's outward procession. Along the processional route, military veterans, NHS workers and others are occupying specially-built viewing stands in Trafalgar Square, at The Mall and in front of Buckingham Palace.

As the rain drizzles down outside Buckingham Palace, the royal fans have come prepared for the "typical British weather."

"There's just a lovely atmosphere – even if it is raining," says Anita Brook, 50, holding her Union Jack umbrella.

"Our first royal event was for the Silver Jubilee in 1977 – I remember the street parties and just fell in love with it. My mum is a massive royalist so we just always loved the royal family – we grew up with this."

Brook travelled to Buckingham Palace with her friend Kashmir Carmichael, 50, from Oxfordshire, in central England.

Wearing a Union Jack poncho, Carmichael remembers when the late Queen Elizabeth visited her school as a child.

"When the Queen came to our school we were so excited. Since she has passed, our favourite royals have to be William and Kate. We haven't missed a royal event since their wedding."

In a slight deviation from tradition, the couple are processing to the ceremony in the Diamond Jubilee State Coach drawn by six Windsor Grey horses. They are being accompanied on the 1.3-mile route by four divisions of the sovereign’s most trusted bodyguards, the Household Cavalry.

Each division consists of 24 horses with two divisions of the Blues and Royals at the front, and two divisions of the Life Guards following.

The procession is being led by the Household Cavalry Mounted Band – that's 48 horses and musicians, who will play eight marches as they travel the route.

The coach’s interior is immaculately upholstered in primrose yellow silk and inlaid with materials tied to Britain and its history.

“It’s a real microcosm of British and world history. There’re woods from the royal residences, from explorations and from other countries and nations as well,” explained Sally Goodsir, the Royal Collection Trust’s curator of decorative arts ahead of today's celebrations.

Royal fans from all over the world have gathered on the streets of London, decked out in flags, crowns and homemade regalia.

But what about for those in the US who haven’t made the trip across the pond?

Here's what you need to know:

After the ceremony, the newly crowned King Charles and Queen Camilla will travel back to Buckingham Palace in the Gold State Coach, which has been used in every coronation since 1831, accompanied by a grand procession and display of British pageantry.

They will then make an appearance on the palace balcony alongside members of the royal family and watch a flypast of 60 aircraft, scheduled to last six minutes.

Haven't kept up with recent royal happenings? Here's a full breakdown of everything you need to know.