LONDON (July 2) - Twenty years after a scruffy one-hit wonder first demonstrated his gift for lofty dreams and grandiose statements, hundreds of the world's top performers and more than 1 million fans united for 10 free concerts across the globe aimed at fighting African poverty.

Bob Geldof claimed Saturday's shows would be ''the greatest concert ever,'' and it was hard to argue with him after the unprecedented gathering drew everyone from Snoop Dogg to Bill Gates, Mandela to Madonna.

But the ultimate success of the LIVE 8 extravaganza will be judged by whether the world's most powerful leaders, gathering next week for the Group of Eight summit meeting, listen to Geldof's demands for debt forgiveness, trade concessions and $25 billion in aid for Africa.

''History and the generations to come will judge our leaders by the decisions they make in the coming weeks,'' former South African president Nelson Mandela said after taking the stage in Johannesburg, where the crowd of more than 8,000 people gave him a five-minute ovation. ''I say to all those leaders: Do not look the other way, do not hesitate ... It is within your power to prevent a genocide.''

''This is our moment. This is our time. This is our chance to stand up for what's right,'' U2 frontman Bono told a crowd of 200,000 in London's Hyde Park.

''We're not looking for charity, we're looking for justice,'' Bono said. ''We cannot fix every problem, but the ones we can, we must.''

In Philadelphia, on the Independence Day weekend, actor Will Smith called the festivities a worldwide ''declaration of interdependence.''

''Today we hold this truth to be self-evident: We are all in this together,'' Smith said. Beamed around the world by satellite, he led the audience in snapping their fingers every three seconds, signifying the child death rate in Africa.

Paul McCartney and U2 opened the flagship show of the free 10-concert festival with a rousing performance of ''Sgt. Pepper's Lonely Hearts Club Band.'' A thunderous roar erupted from the crowd of about 200,000 as icons McCartney and Bono belted out the first line: ''It was 20 years ago today...'' - a nod to Geldof's mammoth Live Aid benefit that raised millions for African famine relief in 1985.

Bono, dressed in black and wearing his trademark wraparound shades, wrapped the crowd around his finger, enticing tens of thousands to sing along to the anthemic ''One'' and ''Beautiful Day.'' The crowd cheered when a flock of white doves was released overhead.

Geldof appeared onstage to introduce Microsoft billionaire and philanthropist Gates, whom the crowd greeted with a rock star's roar.

''We can do this, and when we do it will be the best thing that humanity has ever done,'' Gates said.

The crowd joined in as REM sang ''Man on the Moon,'' then heard U.N. Secretary-General Kofi Annan declare: ''This is really the United Nations ... The whole world has come together in solidarity with the poor.''

Geldof's claim that 3 billion people around the world were watching Saturday seemed overblown, as did talk in Philadelphia that a million people were on hand. But LIVE 8 was huge nonetheless, with a mile-long crowd stretching from the front steps of the Philadelphia Museum of Art and America Online saying that more than 5 million people sampled its live video streams, which broadcast all 10 concerts in their entirety.

The first concert kicked off in Japan, where Bjork and Good Charlotte joined local bands for a show that failed to generate much interest in Asia's only G-8 nation. Despite Bjork making her first live performance in two years, the crowd of 10,000 people was only half of what the hall in the Tokyo suburb of Makuhari could hold.

Still, ''we believe passionately in what this is about,'' Bjork said. ''Just the acknowledgment of the problem is an important step.''

LIVE 8 then rolled on to Johannesburg. That show, plus one featuring African artists in southwestern England, were organized following criticism that African artists had been left out of an event aimed at their own continent.

''Africans are involved in helping Africa, which doesn't happen too often,'' Cameroonian singer Coco Mbassi said before the England show. ''We're presenting a different image of Africa.''

Near Paris, an eclectic lineup including Italian tenor Andrea Bocelli and Goth-rockers The Cure played to a crowd of 100,000 at the 17th-century Palace of Versailles. Faith Hill and Duran Duran joined Italian stars in Rome for a concert at the ancient Circus Maximus, which was packed with about 200,000 fans.

German crowd-pleasers Die Toten Hosen kicked off Berlin's show - which attracted about 150,000 people - with a string of power anthems while reminding revelers that helping Africa stood above the music.

''This is no rock concert, it's a reminder about next Wednesday,'' singer Campino told the crowds, referring to the G-8 meeting.

Canadian favorite Tom Cochrane started that country's concert with ''Life is a Highway'' before 35,000 roaring fans on a crisp sunny morning in Barrie, Ontario. And in Moscow, where 20 years ago residents heard little or nothing about Live Aid because of tight Soviet information controls, tens of thousands jammed a square in the shadows of the Kremlin.

In London, Madonna performed ''Like a Prayer'' hand-in-hand with Birham Woldu, an Ethiopian woman who as a malnourished toddler appeared in some of the most wrenching footage of the 1984-85 famine. Her life was saved, Geldof said, partly through donations from Live Aid viewers.

As night fell, Sting performed ''Every Breath You Take'' as a message to the G-8 leaders - ''We'll be watching you,'' he sang. The Who belted out their classic ''Who Are You?'' to a backdrop of images of the G8 chiefs.

And the crowd went wild for the reunion of '70s supergroup Pink Floyd - the first time guitarist David Gilmour, drummer Nick Mason, keyboard player Richard Wright and bassist Roger Waters appeared onstage together since 1981.

London concertgoer Tula Contostavlos, 19, said she was there to see Mariah Carey - and to send a political message.

''Obviously some people are here for just music,'' she said, ''but they're forgetting what's important and what they're here for.''