Prince George and Princess Charlotte have been singing at a carol service to honour Queen Elizabeth II.
They joined 1,800 people at the Westminster Abbey event hosted by their mother the Princess of Wales.
King Charles will later deliver his first Christmas Day message since the death of his mother.
Sunday also sees royal family members attend a Sandringham church service, marking a return to traditional royal festivities.
It will be the first time they have spent Christmas at Sandringham since 2019 and the royal standard will fly above the private Norfolk estate.
Actress Catherine Zeta-Jones narrated the Westminster Abbey carol service dedicated to the late Queen.
Introducing the programme, which was aired on ITV1, Kate said she was "thrilled" to return to the abbey for a second year.
Reflecting, she remarked that this year will be the first Christmas without the Queen, who died in September at the age of 96.
She added: "Her Majesty held Christmas close to her heart, as a time that brought people together and reminded us of the importance of faith, friendship and family, and to show empathy and compassion.
"This year, we've invited hundreds of inspiring individuals to the service."
During the service, images of the Queen through the ages appeared on a grand piano during an instrumental version of In The Bleak Midwinter.
The service was attended by many members of the royal family including King Charles III and the Queen Consort, the Countess of Wessex, Princesses Beatrice and Eugenie as well as Zara and Mike Tindall.
Kate was seen at one point with a comforting arm around Charlotte's shoulder, while the Prince of Wales placed a guiding hand on his son George's back.
A Christmas tree in the abbey was decorated with small Paddington Bear decorations – a nod to the Queen's famous Jubilee sketch.
Guests were greeted with atmospheric snowflakes from a snow machine as they arrived at the entrance.
Among the performers were the Westminster Abbey choir, singer Craig David and Alfie Boe and Melanie C who sang a duet, while William and Dame Kristin Scott Thomas delivered readings.
The service was aired on Christmas Eve but took place on 15 December – just hours after the final part of the Duke and Duchess of Sussex's controversial documentary dropped on Netflix.
Elsewhere, the Archbishop of Canterbury is expected to say in his Christmas sermon that the Queen "in obedience to the Christ-child lived a life of service and put her interest after those of the people she served."
The loss of the Queen has also been reflected by the Anglican and Catholic archbishops of Wales in a first joint Christmas message.
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