Prince William and Kate, the Duke and Duchess of Cambridge, have entered St. Paul’s Cathedral to attend the service.

Several minutes after their arrival, Prince Charles, the heir to the British throne, and his wife Camilla, Duchess of Cornwall, also made their way into the central London cathedral.

Once inside the cathedral, Charles and Camilla joined William and Kate to form a royal procession before they took their seats for the service.

Other royal family members who already arrived include the queen's daughter, Princess Anne, and her husband Vice Admiral Timothy Laurence; their children Zara Tindall and Peter Phillips, and Zara's husband Mike Tindall; and Elizabeth's youngest son Prince Edward and his family.

Prince Andrew's children, Princess Beatrice and Princess Eugenie, as well as their husbands, Edoardo Mapelli Mozzi and Jack Brooksbank, were also in attendance.

The royal family is attending the National Service of Thanksgiving -- which includes Bible readings, prayers and hymns intended to give thanks for the queen's 70-year reign -- without Elizabeth herself. Buckingham Palace announced Thursday that the queen would not attend in person after experiencing “some discomfort” at the Trooping the Color parade earlier in the day.

Prince Harry and Meghan, the Duke and Duchess of Sussex, have arrived at St. Paul’s Cathedral, marking the first time the couple has appeared publicly with the royal family in two years.

For the occasion, Meghan wore a Dior Haute Couture trench coat and skirt with matching hat and gloves, and Harry wore a tailored black dress jacket featuring his military medals and pinstripe pants. They were greeted by a guard of honor and then the Very Reverend Dr. David Ison on the cathedral's western steps.

The Sussexes stepped down from their roles as senior working members of the royal family in 2020 and later moved to California, where they live with their two children.

The couple watched the Trooping the Color parade Thursday alongside other royal family members at Buckingham Palace, but did not appear publicly.

The British royal family and more than 400 invitees – including government and faith leaders, teachers, military members, and COVID-19 frontline workers – are gathering at a National Service of Thanksgiving to celebrate Queen Elizabeth II’s 70 years on the throne.

The 96-year-old queen will not attend the service after experiencing “some discomfort” at Thursday’s Trooping the Color parade, according to Buckingham Palace.

Other senior royals are expected to attend, including three of the queen’s four children – Princes Charles and Edward and Princess Anne. The queen’s son, Prince Andrew tested positive for COVID-19 and will not attend, a royal source told ABC News.

The service – led by the Very Reverend Dr. David Ison – is being held at St. Paul’s Cathedral in London, which also hosted services for the queen’s Silver, Golden and Diamond Jubilees.

The service will include readings from the Bible, prayers and hymns that will “give thanks for the Queen’s reign, faith and lifetime of service,” according to the palace.

The Royal Marines’ band and trumpeters from the Royal Air Force and the Household Cavalry, which also performed at Prince Philip’s funeral, will play before and after the service.

A new song, titled “By Wisdom” and composed for the Platinum Jubilee, will also be performed.

Queen Elizabeth II and her grandson, Prince William, joined forces Thursday night for a lighting ceremony symbolizing the impact the queen has had around the world.

The queen appeared outside of Windsor Castle and touched a symbolic globe that set off a chain of lights to London, where William stood outside of Buckingham Palace.

There, the light landed at the “Tree of Trees,” a sculpture made of hundreds of indigenous British trees and plants. The tree is the symbolic culmination of The Queen’s Green Canopy, an initiative for the Platinum Jubilee that invited people across the U.K. to plant a tree.”

“Placing the ‘Tree of Trees’ at the heart of the beacon lighting reflects the Royal Family's long history of championing environmental causes and will pay a fitting tribute to Her Majesty’s 70 years of service to the nation,” Buckingham Palace said in a statement.

As the queen and William were participating in the beacon lighting ceremony, more than 1,500 beacons were set to light up across the U.K. and Commonwealth countries and territories.

Queen Elizabeth II shared a written thank you message on Sunday to mark the end of four days of celebrations for her Platinum Jubilee.

"When it comes to how to mark seventy years as your Queen, there is no guidebook to follow. It really is a first. But I have been humbled and deeply touched that so many people have taken to the streets to celebrate my Platinum Jubilee," Elizabeth wrote. "While I may not have attended every event in person, my heart has been with you all; and I remain committed to serving you to the best of my ability, supported by my family."

"I have been inspired by the kindness, joy and kinship that has been so evident in recent days, and I hope this renewed sense of togetherness will be felt for many years to come," the queen continued. "I thank you most sincerely for your good wishes and for the part you have all played in these happy celebrations."

The queen signed her message with her first name, Elizabeth, followed by the letter R, which stands for Regina, the Latin word for queen.