Catherine, Princess of Wales, is presiding over her annual Christmas carol concert at the historic Abbey, with a congregation comprising dignitaries, stars, and everyday helpers who have supported their local areas.
The large audience will enjoy a selection of carols, music and readings from actors such as award-winning actors.
The central message of the service, held in the atmospheric historic church on the afternoon, is generosity and expressing affection towards one's neighbour.
In the audience are Jonathan and Anna Cordiner, whose young girl was lost to a tumour, and who established a foundation, a support fund, to aid parents with a youngster facing a life-limiting condition.
"The time, care and compassion you give, often without fanfare or recognition, and without any expectation or recognition, make an extraordinary difference to the well-being of people," wrote Catherine in a letter ahead of the carol service.
The royal has been undertaking her own measured re-emergence to public life following her health challenges and treatment, recently giving her initial major address in two years, in which she talked about recognising the "quiet, often invisible work of caring".
Guests arriving at the Abbey can listen to young musicians aided by the charitable trust Future Talent, established in part by the former royal to widen access to musical tuition.
An 18-year-old named Carys Wood, who plays the trumpet and is from southwest Wales, has been helped by the organisation, with awards contributing to instruments, tuition, and transportation expenses - and she said it was "incredible" to be playing at the evening's event.
"Music can be very expensive," she stated. She is now studying at the Royal Academy of Music, and added that "all people deserve the chance" to cultivate their performance abilities.
Eliza, Carys's sister who is 11, who performs on the brass instrument, will likewise attend the Abbey. "I never thought I'd be able to play there in front of so many notable figures," she said.
Among the guests will be individuals spanning multiple eras.
That will include nine-year-old Saphia Turner, recipient of a young hero award, who creates and sells art and completes fundraising tasks to raise money for community pantries.
The Abbey has been adorned with custom-created holiday decorations, with TV cook Mary Berry and florist Simon Lycett helping to make them.
Taking to the stage will be Katie Melua, choristers from Westminster Abbey and a maritime folk band from Cornwall, the Fisherman's Friends, who said the chance to take part was "the icing on the cake for us."
The Together at Christmas carol concert will be televised and shown on ITV1 on Christmas Eve.
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