Bob Geldof has addressed crowds at the Glastonbury Festival in Somerset, urging tens of thousands of fans to "make poverty history".
Festival-goers joined hands and chanted his slogan after the Live 8 organiser appeared on the event's main stage.
All performances stopped to allow artists and crowds to take part.
"I want you to individually believe you can change the condition of the most put-upon and beaten-down people on this planet," Geldof told the crowd.
"I want you to grab the hand of the person beside you," he said.
"Not as some big hippy rock festival thing, but because you want this to happen. Lift your hands and say together, 'make poverty history'."
Founder Michael Eavis (left) introduced Geldof
Geldof's Live 8 concerts, which take place around the world on 2 July, aim to put pressure on world leaders to eliminate poverty in Africa.
They are timed to coincide with the G8 summit in Gleneagles, Scotland. He is also backing a protest in Edinburgh on the day of the summit, 6 July.
Geldof said: "On 6 July we will face down those eight men that can do this thing. This is not a question of money.
"To die of want is an intellectual absurdity and it is morally repulsive. I would ask the people watching this on television to imagine half of this field dying now. And the other half dying tomorrow. And between them, those men at the G8 would have resolved it in 10 seconds."
Geldof was introduced on stage by festival founder Michael Eavis, who was making his first appearance at his festival's Pyramid Stage since he founded the event in 1970.
To huge cheers Mr Eavis demanded that G8 leaders "come up with the dosh to make poverty history".
Many revellers were taken by surprise by Geldof's appearance, and some were left in tears.
Sarah Curtis, 37, of Matlock, Derbyshire, said: "I was prepared to be cynical but I thought he spoke really well. I even thought the sun was going to come through, but it didn't quite."
"Everyone was getting involved. Everyone just stopped and listened as soon as he came on," said 25-year-old Alison Downes of Glossop, Derbyshire.
Jeff Preston, 29, of Richmond, south-west London, added: "He was really powerful. He makes you want to do something straight away."
Geldof's appearance came in the middle of the festival, which lasts until Sunday.
Coldplay are headlining on Saturday, with New Order and Keane also playing, as the Somerset festival gets back on track after Friday's flooding.
Kaiser Chiefs were among Saturday's bands
Emergency crews are remaining vigilant in case the bad weather is repeated. Conditions have stayed overcast during Saturday.
Some 200 tents were submerged in up to eight feet of water on Friday, Somerset Fire and Rescue Service said, adding flooding had been made worse by the amount of rubbish left on the site. 414 festival-goers had been looked after by welfare teams.
"The festival's drainage system was superb but discarded rubbish and clumps of plastic bags were found in drains," a spokesman said.
"There would have been some flooding due to the sheer volume in such a short period of time, but it would not have been so bad if there had been no rubbish."
More camping space
Between two and three million litres of water were pumped out of one area, with specialist high-volume pumping equipment - first used in January's flooding at Carlisle - drafted into action for only the second time in the UK.
Unlucky campers have been issued with new tents, and an area the size of three football pitches has been freed up for more camping space.
Police reported 66 arrests, including two indecent assaults.
Clearing up continues after Friday's flooding at the site
The Avon and Somerset force dealt with 118 drug-related offences - up by more than half on this time in 2004's festival, which police put down to better detection of crimes.
Non-drug offences were down by half to 36.
Meanwhile, a 25-year-old man died after being taken ill at a festival campsite on Friday evening.
Avon and Somerset Police said there were no suspicious circumstances surrounding the death, but added they were "keeping an open mind".