U2 are playing in Brazil as part of their Vertigo world tour

The guitar will be auctioned to raise money for Brazilian President Luiz Inacio Lula da Silva's campaign for all Brazilians to have three meals a day.

Bono met the president on Sunday where they discussed poverty and renewable energy sources.

About 70,000 fans saw U2 in Sao Paulo on Monday. "To beat poverty, we all have to work together," Bono said.

Message

"Martin Luther King didn't just have an American dream, but an Irish dream, a Latin American dream - sing for Peru, for Chile, for Argentina, for Brazil," he told the crowd.

But there were boos from the audience at the mention of Argentina and some jeered when pictures of Lula and US President George W Bush appeared on a large screen behind the band.

Bono (left) met the Brazilian president at the weekend

Bono ended the group's two-hour performance with an appeal for zero poverty and thanks to Lula for his hospitality.

The Brazilian president wants to ensure all Brazilians get three meals a day by the end of his term of office on 31 December this year.

His government also wants to expand Brazil's use of bio-diesel fuel based on tropical fruit seeds, sunflowers and soybeans.

U2 will play another concert at the Morumbi football stadium in Sao Paulo on Tuesday before more gigs in Chile and Argentina.

It is not yet known when and where the guitar auction will take place.