Organiser Geldof promised to deliver "the greatest concert ever".
From Johannesburg to Philadelphia, from Berlin to Tokyo, musicians and fans gathered for a global music marathon to raise awareness of African poverty and pressure the world's most powerful leaders to do something about it at the Group of Eight summit in Scotland next week.
Paul McCartney and U2's Bono rocked London's Hyde Park to kick off the main event in the Live 8 extravaganza rolling around the globe from Tokyo to Toronto, Berlin to Johannesburg.
A thunderous roar erupted from the crowd of about 150,000 as the two iconic performers belted out the first line: "It was 20 years ago today ..." - a nod to the mammoth Live Aid concerts that raised millions for African famine relief two decades ago.
The first concert kicked off in Japan, where Bjork and Good Charlotte joined local bands.
It rolled on to Johannesburg, South Africa, then on to Berlin and to the Eden Project in southwest England for a show featuring African artists.
In an open letter to the G8 leaders, which appeared in several British newspapers on Saturday, Geldof said the summit will disappoint the world if it fails to deliver an extra $25 billion in aid to Africa.
No half-measures or politics
"We will not applaud half-measures, or politics as usual. This must be a historic breakthrough," the letter says. "Today there will be noise and music and joy, the joy of exuberant possibility.
"On Friday there will be great silence as the world awaits your verdict. Do not disappoint us. Do not create a generation of cynics." The G8 conference concludes on Friday.
In Tokyo, Iceland's Bjork made her first live performance in two years. "I feel there is not much hope to change anything, but when I heard that Bob Geldof was going to get all these people together I felt there was maybe 5% hope, and that's better than zero," said Bjork. "But at least the world is listening today and this has to change."
The 10,000 tickets for the venue, the Makuhari Messe arena to the east of the capital, were snapped up in just 36 hours, according to organisers.
Japanese fans called on
their government to do more
But the crowd of 10,000 people was only half of what the hall in the Tokyo suburb of Makuhari was capable of holding.
"Japan is a rich country, so we should do more," Mayumi Kamata, 29, said. "Not many people know the full details, but this event will bring the matter to more people and maybe the govenment can be convinced to change things."
American rock band Good Charlotte also called on their government to do more. "My messge to George Bush is please help," said front-man Joel Madden. "We have such a powerful country but there is so much more that we can do.
Zambian singer Lindiwe opened in Johannesburg before a crowd of about 500 people that was expected to swell to 40,000.
Both the Africa Calling concert at the Eden Project and the Live 8 concert in Johannesburg were organised following criticism that African artists had been left out of the Live 8 concerts, despite the event's aim to raise awareness of the continent's plight.
"Africans are involved in helping Africa, which doesn't happen too often," Cameroonian singer Coco Mbassi said before the Eden Project concert. "We're presenting a different image of Africa - showing that Africa has good things to give."
There are shows Saturday in all the G8 countries - the United States, Britain, France, Germany, Italy, Canada, Japan and Russia - as well as in South Africa, where former president Nelson Mandela was expected to address the crowd in Johannesburg.
Zambian singer Lindiwe opened
the concert in Johannesburg
In Hyde Park, organisers and police geared up for one of the biggest outdoor events ever held in Britain.
More than 150,000 people have tickets for the free show, with another 55,000 expected to watch on giant screens in the park.
More than 1000 police officers will also be on duty. "Police leave for this weekend was cancelled some time ago," said Metropolitan Police Chief Superintendent Helen Ball.
"We are not expecting trouble and there is no intelligence about a particular security threat to this event, but we have planning for a whole range of different things happening," she added.
Thousands of fans streamed into Hyde Park hours before the show - some hedging against the unpredictable British weather by wearing both sun hats and raincoats.
Harvey Goldsmith, one of the Live 8 organisers, said he was determined the show, featuring 25 acts including U2, Madonna, REM, Mariah Carey, Coldplay and Pink Floyd, would go like clockwork.
"We have clocks everywhere and our message to all the artists is, we don't care what time you go on, we just care about what time you come off," he said. "We have to be as time-precise as possible, particularly with all the international links going on. We just ask everybody to play ball."
"We will not applaud half-measures, or politics as usual. Do not disappoint us. Do not create a generation of cynics"
open letter to the G8 leaders
More than a million people may attend the concerts, and organisers say 85% of the world's population will have access to a television, radio or Internet broadcast of the day's events.
A complex broadcasting operation will feed footage of the concerts to networks including the British Broadcasting Corporation (BBC) - which is devoting more than 12 hours on its main TV channels to the event - and MTV.
The lineups are eclectic: Destiny's Child, The Dave Matthews Band, Bon Jovi, Stevie Wonder, P Diddy and Jay-Z in Philadelphia; Brian Wilson, Chris de Burgh and Green Day in Berlin; Neil Young, Bryan Adams and Motley Crue in Barrie, north of Toronto; McFly in Tokyo; the Pet Shop Boys in Moscow; Goth-rockers The Cure and Senegalese superstar Youssou N'Dour in Paris.