In a leaky classroom without desks, Mr Brown heard calls for support, not a magic solution

They had all come to listen and learn from the schoolboy Eugenio Armando, and to launch the most ambitious scheme in the history of world education: a $100bn, 10-year plan to give every child in the world a place at school. It would mean 100 million more children getting a primary education.

Eugenio, 12, is one of the lucky ones: he is at school, the Escola Primaria des Forces Populares de Liberacao de Mocambique in Maputo. Trouble is, there are 4,000 others at the school, and only 70 teachers. The children are taught in shifts, and classes have to be abandoned when daylight dims.

Eugenio told Mr Brown: "The rain comes in and exercise books get wet. Most classrooms don't have any desks. I know you will not be bringing magic solutions to our problems but we hope you will be able to mobilise support to help us."

And that is the Chancellor's aim. As more than 60 children sat in lines on the dusty classroom floor chanting their vowels, Mr Brown outlined his vision to bring international pressure to bear on the world's richest nations. He wants them to deliver on pledges of huge increases in aid spending made at the G8 summit in Gleneagles last year.

He said: "It looks as though it ought to have 300 or 400 pupils, but there are more than 4,000. There ought to be several hundred teachers, but there are only about 70. The pupil-teacher ratio is about 70 to one."

Children at the school are taught in three-hour shifts to cope with the numbers. Classes can go on into the evening, but are often cut short because of failing light.

Mr Brown said he had been inspired by the sight of packed classrooms to pursue his drive to bring primary education within the reach of children everywhere. He unveiled a £7.5m programme to send 4,000 teachers and 2,000 children from the UK on exchanges to developing countries in Africa, Asia and Latin America between now and 2009.

He said that he would try to forge links between schools that would represent "the world's largest school assembly". Schools will be able to apply for grants of £1,500 to send teachers and their teenage pupils on 10-day exchanges to far-flung regions of the world.

Mr Brown said: "This is not just one-way traffic. These children have not got tables, they have not got chairs or even roofs but they still want to learn. We can learn from their enthusiasm for education."

Mozambique yesterday agreed to draw up a 10-year plan for education, alongside states such as Nigeria and Tanzania. The country has put one million more children into school since 2001 and built 1,840 schools. But about one million children in the country still do not go to school, some of the 100 million worldwide denied the sort of education taken for granted in the UK.

The Chancellor won a rare public appearance from the former South African president Nelson Mandela to endorse his plans to secure $10bn a year from the world's richest nations.

The frail 85-year-old indicated that this appearance may be among his last. He said: "I'm truly in retirement. This was an exception."

He answered questions from two British schoolchildren, Lily King Taylor, 13, and Jenade Sharma, 12, from Newham, east London, who travelled to Mozambique with Comic Relief.

Mr Mandela said: "The message you should take back is you can make a difference via your actions. You might think you are powerless, but if all the children of Britain act together you can be more powerful than any government."