Coldplay singer Chris Martin joined US band REM on stage at the climax of Oxfam's Make Trade Fair benefit show in Hammersmith, London, on Tuesday.
REM mixed old hits and new songs during their unannounced set
The British star joined REM's Michael Stipe on vocals for Man on the Moon during their "surprise" headline set.
The event also featured Jamelia, The Thrills, actress-turned-singer Minnie Driver and comedian Ricky Gervais.
Coldplay have been big supporters of Make Trade Fair, which calls for reform of world trade laws to reduce poverty.
"I was introduced to the idea of fair trade because I'm a massive Coldplay fan, so I'd like to thank Chris Martin," Stipe told the crowd.
Speaking at a press conference before the event, Driver said fair trade was "not the sexiest of campaigns".
But the Oscar-nominated actress - whose debut album is released in October - said musicians had "a great platform to educate and encourage kids to take an interest in what's going on globally".
Oscar-nominated actress Minnie Driver is launching a singing career
"I don't think kids know poverty is the most preventable cause of death in the world, and trades will go a long way to alleviate it," she said.
"It's brilliant to actually do something, not just talk about it. To get off your arse and do something is great."
Her sentiments were echoed by Jamelia, whose set included a cover version of the Blondie song Heart of Glass with Razorlight singer Johnny Borrell.
"It's a worthy cause to be supporting, and if there are a few people who find out among themselves about this, then that would be a good thing," said the 22-year-old R&B star.
The Make Trade Fair Live event followed on from Oxfam's Fair Play concert in 2002, which saw Chris Martin, Noel Gallagher and Ms Dynamite perform on stage.
This year's concert was compered by TV presenter Dermot O'Leary.