A 30th anniversary version of the Band Aid charity single was recorded by Bob Geldof, One Direction, Bono and some 30 other stars to raise money to fight Ebola.
Led Zeppelin's Robert Plant, Coldplay's Chris Martin and Sinead O'Connor were also among the rockers brought together by Geldof to sing the fourth version of "Do They Know It's Christmas?"
Musicians began arriving in the early morning and were expected to record all day and into the night before the single is aired for the first time on Sunday and then officially released on Monday.
The Ebola outbreak has claimed more than 5,000 lives since last December, according to the World Health Organization -- almost all in Liberia, Guinea and Sierra Leone -- while the number of infected cases registered worldwide has soared to more than 14,000.
"All pop singers can do is sing, write songs, give up whatever they're doing," Geldof told reporters and fans outside the Sarm Studios in Notting Hill.
He said owning the single would be "a badge of honour" to support efforts against the deadly virus and urged people to buy it "whether you like it or not".
Making his way into the studio, U2 frontman Bono hit out at the response of rich countries, saying if they "kept the promises they make at these big G8 meetings and the like we wouldn't have to be standing here."
One Direction's Niall Horan said it was a "privilege" to take part.
"Hopefully we'll get to number one and raise a lot of money for a really good cause," he said.
In a video from inside the studios that was put up on a specially created app called "Bandapp", One Direction's 20-year-old frontman Harry Styles said: "Everyone's come together for the same thing."
"There's one kind of focus and one goal," he said.
The song became one of the world's biggest-selling singles ever after its release in 1984 to raise funds for famine relief in Ethiopia.
"I had to change the lyrics," Geldof said on the new version.
The rocker-turned-activist said he had been spurred into action not out of nostalgia but by a call from the United Nations, concerned about not having the necessary funds to combat the epidemic.
- 'Lovers can't hold each other' -
"It's not just about what's happening in west Africa, it could happen here tomorrow," Geldof said.
"We can stop this thing, we can allow mothers no matter where they are to be able to touch their dying children.
"I hate that aspect of it that lovers can't hold each other in their last moments, that husbands can't comfort their wives, that parents can't comfort their children," Geldof said.
The second and third verses of the song contain new lyrics referring to the risks of cross-infection from comforting Ebola victims.
The track will cost 99 pence ($1.60, 1.30 euros) to download or Ģ4 to buy as a physical record.
Geldof said he had spoken to British finance minister George Osborne on Saturday who agreed not to levy sales tax from the single to ensure that 100 percent of the proceeds go to charity.
"He said: 'I've been expecting this call. It's a bit early for pop stars isn't it?'."
The single is already a hot favourite to claim the prestigious Christmas number one spot in Britain, with bookmakers Coral quoting odds of 4-6.