Japan has deployed some 20,000 police to seal off the summit venue
Rising food and fuel prices are topping the agenda for leaders of the world's major industrialised nations as they start a three-day summit in Japan.
Group of Eight (G8) leaders held talks with African leaders on the first day.
UN Secretary General Ban Ki-moon called on the G8 to tackle the "interconnected challenges" of rising food prices, development, and climate change.
Many Western leaders have also used the summit to express concern about the situation in Zimbabwe.
The G8 summit opened at a resort on the northern island of Hokkaido.
Leaders from member nations - Britain, Canada, France, Germany, Italy, Japan, Russia and the United States - will be joined by counterparts from some 15 other countries, including eight African states.
Japan has spent a record sum of money and deployed about 20,000 police to seal off the remote lakeside town of Toyako for the three-day talks.
As the summit began, the UN secretary general urged donor nations to keep their promises, to help poorer countries achieve the UN Millennium Development Goals.
Mr Ban also told reporters that urgent action was needed to guarantee food safety.
"High food prices are already turning back the clock on development gains," he said.
The impact on the global economy of price rises and other shocks such as the credit crunch have eclipsed other concerns, correspondents say.
The BBC's political editor, Nick Robinson, who travelled to the summit with the UK prime minister, says Gordon Brown will join other leaders in calling for the doubling of food production in Africa.
Our correspondent says the G8 may call for the creation of a panel of international experts to advise on how to predict and avoid another crisis like this.
The EU has already been spelling out plans to alleviate the food crisis.
European Commission President Jose Manuel Barroso told reporters on the sidelines of the summit that the proposed 1bn euro ($1.6bn, £800m) fund to help poor farmers in developing countries would come from unused EU subsidies.
It could help improve farmers' access to seeds and fertilisers, and could provide "safety net measures for the most vulnerable", he said.
The G8 leaders may also face tough questions on aid commitments to Africa. Three years ago they promised to double aid to the continent by 2010 - but campaigners say they are falling far short of that target.
As well as discussing development issues in Africa, the G8 leaders have been discussing Robert Mugabe's controversial re-election in Zimbabwe last month.
US President George W Bush said: "I am extremely disappointed in the elections which I labelled a sham election."
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Tanzanian President Jakaya Kikwete, who is also head of the African Union, said the whole continent shared President Bush's concerns but that there was some disagreement over what to do about it.
President Kikwete called for a unity government, said discussions would continue and was optimistic that, "as friends at the end of the day we'll come to an understanding". A number of other bilateral meetings are taking place on the sidelines of the summit.
Mr Bush, attending his last G8 summit, and Russian President Dmitry Medvedev, attending his first, made little progress on the issue of the US plan for missile defence installations in the Czech Republic and Poland.
The two leaders instead cited areas where they had found common ground - preventing Iran from obtaining nuclear weapons and ending North Korea's nuclear weapons programme.
Mr Brown will also have talks with Mr Medvedev.
Meanwhile, the charity Water Aid has told the G8 that the single most effective measure it could take to prevent the deaths of millions of children in poor countries would be to build toilets and provide clean water.
Hundreds of protesters again marched through Sapporo on Sunday, the city closest to the venue, to demand G8 leaders take action on global warming, poverty and rising food prices. The demonstration, which followed a similar protest on Saturday, was heavily policed and ended peacefully.
Violent anti-globalisation marches have marred past G8 meetings.
As the G8 got under way in Japan, leaders of the world's largest Islamic nations assembled for what is being billed as the D8 summit in Malaysia's capital, Kuala Lumpur, with the issue of inflation also high on the agenda there.