Stars including One Direction, Emeli Sande, Rita Ora and Ed Sheeran have recorded a new version of the Band Aid charity single in a studio in London.
Ellie Goulding, Sam Smith, Chris Martin and Bono also sang on the reworked Do They Know It's Christmas?
Sande said the participants "sounded like a really powerful choir" when they sang the chorus together.
The recording, which comes 30 years after the original Band Aid, will raise money for the fight against Ebola.
The UK government has agreed to waive VAT on the single.
Organiser Bob Geldof, who also masterminded the original in 1984, said he addressed the participants "like the headmaster" before they sang the chorus.
"I said, 'OK, this is what it's about. When you sing this, be aware that the rest of the world will be singing it with you.'
"I explained the situation in West Africa, I explained what the UN were saying, explained what we could do, and just geed them up."
Emeli Sande said: "Bob gave a really touching speech before we started singing so I think that really got everybody in the mind frame that we needed to be in to remind us it's fun but we're here for a really serious reason.
"It was him expressing how passionate he was about this cause and how it's so unnecessary that so many people should be dying."
Paloma Faith said: "The most emotional moment was probably when Bob was giving his speech before we sang. I think that injected a lot of raw emotion into the performance."
The song's first lines have been recorded by One Direction. It will get its first play on ITV's X Factor results show on Sunday.
Bono, who also appeared on the original, told BBC News: "We've been trying to make this event a thing of the past. We want to make Band Aid history.
"If every country had the values of Great Britain and kept the promises they'd made at these big G8 meetings, we wouldn't have to be standing here.
"I just wish we didn't have to do this. There will come a time when we won't. And it will be great not to see the same old faces."
Other artists taking part included Elbow, Seal, Jessie Ware, Fuse ODG, Sinead O'Connor, Angelique Kidjo, Olly Murs, Paloma Faith, Queen drummer Roger Taylor, Clean Bandit and Foals.
Rita Ora was the first to record her lines and left early so she could travel to Salford to film BBC One's The Voice.
Ed Sheeran flew in from his tour in Germany, while Bastille cancelled two US arena gigs to be involved. Disclosure and Underworld were also in the studio to do remixes.
Saturday's recording session took place at the Sarm studios in west London, the venue where stars such as Wham!, Duran Duran, Sting and Status Quo recorded the original.
That single raised £8m for famine relief in 1984. This time, the lyrics have been rewritten.
Instead of the line "Where the only water flowing is the bitter sting of tears", the new version has the line "Where a kiss of love can kill you and there's death in every tear".
Bono's famous line "Well tonight thank God it's them instead of you" has been changed to "Well tonight we're reaching out and touching you".
The music video will be edited in "record time" so it can be shown on The X Factor at 20:00 GMT on Sunday.
The song will be made available to download at 08:00 GMT on Monday, priced 99p. A CD version costing £4 will be released three weeks later with a cover designed by artist Tracey Emin.
The track will not be made available on Spotify and other music streaming services until January.
Geldof told fans: "Come Monday at 08:00 when every station plays this track across the nation, buy this thing. Don't go looking for it free. Buy it."
The original version sold 3.7 million copies. Five years later, another version was produced by Stock, Aitken and Waterman, largely featuring their artists such as Kylie Minogue, Jason Donovan and Sonia.
In 2004, Band Aid 20 was recorded by the likes of Coldplay, Dizzee Rascal, Ms Dynamite, Will Young and Robbie Williams.
Band Aid 30 lyrics
It's Christmas time, and there's no need to be afraid
At Christmas time, we let in light and banish shade
And in our world of plenty, we can spread a smile of joy
Throw your arms around the world
At Christmas time
But say a prayer, pray for the other ones
At Christmas time, it's hard but while you're having fun
There's a world outside your window, and it's a world of dread and fear
Where a kiss of love can kill you, and there's death in every tear
And the Christmas bells that ring there are the clanging chimes of doom
Well tonight we're reaching out and touching you
No peace and joy this Christmas in West Africa
The only hope they'll have is being alive
Where to comfort is to fear
Where to touch is to be scared
How can they know it's Christmas time at all
Here's to you
Raise a glass to everyone
And here's to them
And all their years to come
Let them know it's Christmas time after all
Feed the world, let them know it's Christmas time again
Feel the world, let them know it's Christmas time again
Heal the world, let them know it's Christmas time again