LONDON — Hundreds of the world's top performers and more than 1 million fans united yesterday for 10 free concerts across the globe aimed at fighting African poverty.

Bob Geldof, whose Live Aid two decades ago raised millions for African famine relief, claimed the 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 this 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 in Johannesburg, where the crowd of more than 8,000 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.

In Philadelphia, actor Will Smith called the festivities a worldwide "declaration of interdependence."

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 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 Live Aid benefit in 1985.

Geldof appeared onstage to introduce Microsoft billionaire and philanthropist Gates, whom the crowd greeted with a 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."

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.

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.

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

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

Canadian favorite Tom Cochrane started that country's concert with "Life is a Highway" before 35,000 roaring fans in Barrie, Ontario. And in Moscow, 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.

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 "Who Are You?"

And the crowd went wild for the reunion of Pink Floyd — the first time David Gilmour, Nick Mason, Richard Wright and Roger Waters had appeared onstage together since 1981.