Many politicians were among the guests, including Prime Minister Boris Johnson and his wife Carrie.
Among the first members of the Royal Family to arrive were Zara Tindall, the daughter of Princess Anne, and her husband Mike Tindall.
Prince Andrew was not at the service because he has Covid, but both his daughters were there. Princess Beatrice arrived with her husband Edoardo Mapelli Mozzi.
Princess Eugenie arrived with her husband, Jack Brooksbank.
Soon after, Prince Harry and Meghan, Duchess of Sussex, arrived at the service. It was the first time they have been seen in public since coming to the UK from their home in California to attend the Platinum Jubilee celebrations.
They were followed by Princess Anne and her husband Tim Laurence, and Prince Edward and his wife Sophie, Countess of Wessex and their children.
Prince William and his wife Catherine, Duchess of Cambridge, came without their children.
The last guests to arrive were the Prince of Wales and his wife Camilla, Duchess of Cornwall.
Once the service started, Mr Johnson gave a reading.
The most senior members of the Royal Family sat together at the front.
The Wessex family, Princess Eugenie and Princess Beatrice and Prince Harry, sat across the aisle.
After the service, the family left in a procession.