Nelson Mandela rallied thousands in Trafalgar Square to fight poverty, an evil on par with "slavery and apartheid".

On Thursday the former South African president said poverty is "man-made" and can be defeated.

He urged world leaders to "not look the other way", but to deliver fair trade, debt relief and more and better aid. "Recognise that the world is hungry for action, not words."

Looking frail and leaning on his stick, the 86-year-old said: "I recently formally announced my retirement from public life and should really not be here.

"However, as long as poverty, injustice and gross inequality persist in our world, none of us can truly rest.

"Massive poverty and obscene inequality are such terrible scourges of our times that they have to rank alongside slavery and apartheid as social evils.

Police estimated up to 20,000 people filled the square where ant-apartheid protests were held before Mr Mandela's release from prison in 1990.

"Overcoming poverty is not a gesture of charity," the Nobel peace prizewinner added. "It is an act of justice. It is the protection of a fundamental human right, the right to dignity and a decent life.

He gave a white band, the symbol of the anti-poverty campaign, to a group of British school children.

"Sometimes it falls upon a generation to be great," he told them. "You can be that great generation. Let your greatness blossom.

"Of course the task will not be easy. But not to do this would be a crime against humanity, against which I ask all humanity now to rise up. Make Poverty History in 2005."

The Make Poverty History campaign, which invited Mr Mandela to Britain, is a coalition of more than 200 charities.

Britain has made Africa a top priority of its presidency of the Group of Seven (G7) industrialized nations this year.

When G7 finance ministers meet this weekend in London, Chancellor Gordon Brown will propose a "Marshall Plan" for Africa. After World War II, America used the Marshall Plan to reconstruct Europe.

Mr Mandela backs the scheme. He will have a "fireside chat" with the G7 leaders before the start of their talks tomorrow. Debt relief is one of the items on the weekend's agenda.

Live Aid crusader Sir Bob Geldof, who introduced Mr Mandela as "the president of the world", said: "I'm tired of the politics of being nice. I want the politics of responsibility.

"I'm sick of standing in squares, linking arms for a cause, tear gas and records, I'm sick of this."

Sir Bob repeated the Live Aid slogan that has become world famous after 20 years. "I say again, Feed The World, for we starve for justice, hunger for dignity, thirst for an end to degradation, and we have nothing to nourish our dreams."

Five schoolgirls from Norfolk and Suffolk got up at 5am to join the rally. Poverty is a "big issue", believes 16-year-old Georgina Pearson. "People need to do something about it."

Their posters asked simply: "What if it was you (sic)?"