A preliminary World Bank study released last week estimated that up to 105 million more people could drop below the poverty line due to rising food prices, including 30 million in Africa.
In Liberia, the cost of food for a typical household jumped by 25 percent in January alone, increasing the poverty rate to over 70 percent from 64 percent, the study found.
Max Lawson, a policy adviser to Oxfam, said the summit was arguably the most important G8 gathering in a decade.
"The world is clearly facing multiple crises - serious, serious economic problems, both rich and poor countries. But it is poor people who suffer the most, suffering hugely from food price increases," Lawson told reporters.
ZIMBABWE DIVISIONS
World leaders took the opportunity to raise the prospect of more sanctions against Zimbabwe unless quick progress is made to end a political crisis after a run-off election in June in which Robert Mugabe was the only presidential candidate.
The opposition candidate withdrew amid widespread violence against his supporters.
They told the African leaders at the gathering to deal with Mugabe or trade and investment on the whole continent would be affected, a Canadian official told reporters.
Tanzanian President Jakaya Kikwete suggested that African leaders shared the G8's concerns, but differed over the best response.
The G8 comprises the United States, Japan, France, Britain, Germany, Canada, Italy and Russia.
Many critics and even member countries suggest the G8, formed in 1975 with just six members in the wake of the first oil crisis, should expand to take in large developing nations to better represent the world.
On Monday hundreds of demonstrators from Japan and other countries marched in heavy rain toward the summit venue, carrying signs slamming the rich nations' cosy club.
Heavy security meant that they were kept several kilometres (miles) away. Two groups tried to take unauthorised routes but were turned back by dozens of police.
Global warming will be the focus of an expanded meeting on Wednesday that will include China and India, two fast-growing economies that are pumping out more and more greenhouse gases.
But deep divisions within the G8 as well as between rich and poor nations have raised doubts about the chances for progress beyond last year's summit, where the G8 agreed to "seriously consider" a global goal of halving greenhouse gas emissions by 2050.
The European Union and green groups are piling pressure on a reluctant United States to agree to a target to halve global greenhouse gas emissions by mid-century and back the need for 2020 targets for rich countries as well.
