A campaign group has called the Live 8 London concert "hideously white" for not having enough black performers.
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Superstar Mariah Carey is almost the only ethnic minority performer at Hyde Park on 2 July, one of five global Live 8 concerts that day.
London-based group Black Information Link said organisers had "handpicked a virtually all-white line-up".
But a Live 8 spokesman said: "Bob Geldof approached a number of urban and black artists to participate."
He added a number were busy with other projects and commitments.
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The concerts aim to raise pressure on world leaders to fight poverty in Africa, echoing Bob Geldof's Live Aid concerts 20 years ago.
But Patrick Augustus, a black musician and author of BBC drama Babyfather, said black artists had been "totally excluded" from the London concert.
"It seems like the great white man has come to rescue us while the freedom fighters never get a mention," he wrote on the Black Information Link website.
He said Live 8 organisers "need to engage British, African and Jamaican artists who have been dealing with these subjects for a while".
He added: "Where are the reggae artists that have been campaigning for truth and justice over all these years?"
Justin Onyeka, entertainment editor at black newspaper New Nation, said he was surprised Band Aid 20 performers Dizzie Rascal, Ms Dynamite, Jamelia and Beverley Knight were not included in the line-up.
"It was the same problem 20 years ago when major black artists were backing singers to other acts," he said. "It's time they learnt from previous experience."
The Live 8 spokesman added: "We are not doing a show purely to entertain, we are doing a show to appeal to a mass audience to raise awareness of poverty in Africa.
"We look upon Live 8 as one global concert. A number of urban acts in the UK are hugely talented but they are not well known in Paris or Rome."
Further performers were being asked to participate in addition to those already announced, he said.
Meanwhile Live 8 organisers face a large VAT bill because a portion of the money raised in its text message ticket lottery will be spent organising the event.
Organisers pledged £1.6m to the Princes' Trust charity from money made in a text message ticket competition to win 75,000 pairs of tickets to the London concert.
However they are due to be taxed on the remaining money made from the competition, to be launched on Monday.
The spokesman said: "After the concerts we will start discussions to see if we can get back some of the VAT we were charged. That is what Bob is good at."