Nelson Mandela uttered just four words criticizing Zimbabwe's leadership, but they were enough to resonate around the world.

The elder African statesman is in London to celebrate his upcoming 90th birthday, including meetings with the prime minister and the queen, dinner with A-list celebrities and a star-studded concert for nearly 50,000 fans on Friday.

That gig now looks like being overshadowed by events in Zimbabwe, where widely condemned elections are due to be held on the same day.

Although basking in the adoration he traditionally enjoys on visits abroad, Mandela, a man seen as a rare moral authority, has been under increasing pressure during his stay in Britain to condemn Zimbabwe's President Robert Mugabe.

In a speech at a dinner late on Wednesday, Mandela highlighted several global crises including poverty, the Middle East, Iraq and Darfur before referring to a "tragic failure of leadership" in Zimbabwe.

While brief, the phrase hit the headlines, with reports saying Mandela had "broken his silence". He officially retired from politics nine years ago.

Friday's concert has been organized to raise funds for Mandela's AIDS/HIV charity called 46664, named after the prison number he was given during more than a quarter of a century behind bars for his stand against South African apartheid.

The crowd in London's Hyde Park is expected to number 46,664 people who will hear the likes of Annie Lennox, Queen, Leona Lewis and Amy Winehouse performing.

Winehouse's spokesman said on Thursday the Grammy-winning soul singer was due to appear despite being diagnosed with a "touch of" lung condition emphysema earlier this month.

SPEAKING OUT "MAY NOT HELP"

It gives Mandela a far more public platform to expand on his comments about Zimbabwe, but even if the 89-year-old is tempted to take the stage, some of the acts singing on the night argue such an approach could backfire.

"That would not help," said Emmanuel Jal, a Sudanese hip-hop artist based in London. "The more we finger point him (Mugabe), the more the people in Zimbabwe suffer.

"For me, Nelson Mandela has been wise enough over the years and knows the press may twist things," he told Reuters.

"The only way to save Zimbabwe is if he travels to Zimbabwe. Let it be a secret meeting, do things underground, find out what is in his (Mugabe's) mind."

Jal, who was a child soldier in Sudan's ethnic conflict before he was rescued by an aid worker, will be performing "Emma", a song from his new album "Warchild".

He said it was a tribute to Emma McCune, who smuggled him into Kenya when he was 12. She died in a car crash a year later.

"Emma rescued over 150 child soldiers and one happened to be me and I never had a chance to say thank you to her," Jal said.

He added that Mandela was "unique" among African leaders.

"He did not love power so much, and left it and gave it to someone else."

Friday's gig also marks the 20th anniversary of another Nelson Mandela tribute concert held at Wembley Stadium in London, where Simple Minds, Sting and Stevie Wonder were among those lobbying for Mandela's release from prison.