Catherine, Princess of Wales, is presiding over her customary Yuletide celebration at the iconic London venue, with a congregation comprising members of the royal family, famous faces, and community champions who have supported their local areas.
The 1,600 guests will listen to a blend of carols, music and readings from stars like award-winning actors.
The central message of the service, held in the atmospheric historic church on the afternoon, is compassion and demonstrating care for others.
In the audience are Jonathan and Anna Cordiner, whose young girl was lost to a tumour, and who set up a charity, a support fund, to help households where there is a youngster facing a life-limiting condition.
"The dedication, empathy, and kindness you show, often quietly and unspoken, and without any hope for reward or acknowledgement, make an remarkable change to the circumstances of individuals," wrote Catherine in a note prior to the concert.
The royal has been undertaking her own steady comeback to official duties following her health challenges and treatment, in November making her major oration since 2023, in which she talked about recognising the "quiet, often invisible work of caring".
Attendees entering the Abbey will enjoy performances from budding artists backed by the Future Talent organisation, set up with the help of the former royal to increase availability of music lessons.
Carys Wood, aged 18, who performs on the trumpet and is from southwest Wales, has been helped by the trust, with funding assisting with equipment, teaching, and travel fees - and she said it was "awesome" to be playing at tonight's service.
"Pursuing music is often costly," Carys said. She is now enrolled at the Royal Academy of Music, and added that "everyone should have the opportunity" to hone their artistic talents.
Her younger sibling Eliza, aged 11, who performs on the brass instrument, will also be at the Abbey. "I never imagined I would perform in front of so many distinguished guests," she said.
Attending the service will be representatives of different generations.
That will feature young Saphia Turner, nine, winner of a youth community award, who creates and sells art and completes fundraising tasks to raise money for local food charities.
The Abbey has been festooned with custom-created holiday decorations, with TV cook Mary Berry and florist Simon Lycett assisting in their creation.
The musical lineup includes Katie Melua, the Abbey's own choir and a nautical singing ensemble, Fisherman's Friends, who said the chance to take part was "the perfect culmination."
The festive event titled 'Together at Christmas' will be broadcast on a major network on Christmas Eve.
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