Catherine, Princess of Wales, is presiding over her customary Yuletide celebration at the historic Abbey, with a congregation comprising members of the royal family, famous faces, and community champions who have helped their communities.
The sizable congregation will listen to a mix of carols, music and readings from stars like award-winning actors.
The theme of the concert, held in the atmospheric historic church on Friday, is compassion and demonstrating care for others.
Attending the event are a couple named the Cordiners, 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 dedication, empathy, and kindness you show, often quietly and unspoken, and without any expectation or recognition, make an extraordinary difference to the lives of others," Her Royal Highness stated in a note prior to the carol service.
The Princess of Wales is continuing her own measured re-emergence to the public eye following her health challenges and treatment, in November making her first public speech for two years, in which she discussed recognising the "discreet, frequently unseen labour of caregivers".
Attendees entering the Abbey will enjoy performances from young musicians aided by the charitable trust Future Talent, established in part by the Duchess of Kent to broaden opportunities for instrumental training.
An 18-year-old named Carys Wood, who is a trumpet player and is from southwest Wales, has been supported by the charity, with funding assisting with musical instruments, lessons and travel costs - and she said it was "awesome" to be taking part in the concert.
"Musical pursuits are frequently pricey," Carys said. She is now studying at the prestigious music academy, and added that "everyone should have the opportunity" to develop their musical skills.
Her younger sibling Eliza, aged 11, who is a euphonium player, will also be at the Abbey. "It's unbelievable that I get to perform here in front of so many important people," she said.
Among the guests will be people from various age groups.
That will include nine-year-old Saphia Turner, winner of a prize for youthful altruism, 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 celebrity baker Mary Berry and floral designer Simon Lycett helping to make them.
The musical lineup includes recording artist Katie Melua, the Westminster Abbey choristers and a Cornish sea shanty group, Fisherman's Friends, who said the invitation to sing was "the perfect culmination."
The Together at Christmas carol concert will be televised and shown on a major network on Christmas Eve.
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