Genoa's streets are deserted as police await the protesters
They will be joined on Saturday by Russia, making it a Group of Eight (G8) meeting.
The leaders are in part responding to a surge of protest against globalisation, and thousands of demonstrators are expected to take to the streets in Genoa on Friday.
The Italian police have sealed off the city, and 20,000 police are standing by to deal with the protests.
US President George W Bush criticised the demonstrators, saying that they have "embraced policies which would lock the world's poor into poverty,"
Meanwhile, the world leaders have invited representatives of developing countries to join them on Friday evening, when they plan to announce a $1bn global health fund to fight Aids, TB and malaria.
Trade tensions
The world leaders are also expected to endorse calls for a new round of world trade talks, and offer concessions to developing countries, many of whom have been reluctant to take part.
The UK's Prime Minister, Tony Blair, is among those who will argue that an increase in free trade is the best way to help the world's poor.
But the meeting is unlikely to resolve the serious differences between the United States, Europe and Japan on the scope and purpose of the trade negotiations - which have been stalled since December l999 when mass protests and similar disagreements disrupted plans for their launch.
Mike Moore, the head of the World Trade Organisation, has warned that unless agreement on the agenda is reached by the end of July, it will be difficult to launch a successful trade round.
Trade talks are scheduled to begin in Doha, Qatar, in November.
World recession
The leaders will also consider the problems of the world economy, which is slowing down dramatically.
Recession in Japan and a sharp slowdown in the United States are spreading to Europe, threatening global trade and growth.
The US central bank, the Federal Reserve, has already cut interest rates seven times in an attempt to boost the flagging economy.
The United States is expected to press European leaders to do more to stimulate their own economies, including urging the European Central Bank to cut European interest rates further.
But markets have low expectations of any concrete measures arising from Genoa.
Most believe that neither the US - with its new, free market, hands-off approach - nor Europe has the political will to tackle global imbalances, such as the over-valued dollar.
Serious disagreements
There are expected to be deeper disagreements among the leaders about the US position on global warming and missile defence, which will be on the official agenda on Saturday.
Environmental campaigners are furious that US President George W. Bush rejected the Kyoto treaty in March.
They accuse him of putting the interests of America business above those of the planet.
Mr Bush occupies a similar hate figure role for those who bitterly oppose his plans for anti-missile defence - plans that have proven unpopular with many of his fellow G8 leaders too.
Violent protests
Over 100,000 demonstrators are expected, and some of the protest could be violent.
The more militant protesters, led by Italy's Tute Bianche, the White Overalls, have vowed to breach the summit's security perimeter and get into the secure Red Zone.
They said they would make their first attempt during Friday's opening ceremonies.
The Genoa Social Forum (GSF), an umbrella group for around 700 protest groups, has been schooling protesters in self-defence ahead of expected clashes with the security forces.
Protesters divided
The demonstrators themselves are divided on tactics - peaceful campaigners say the violent few are grabbing the spotlight and diverting attention from serious arguments.
But the authorities are taking no chances and are staging the most elaborate security preparations in the history of these annual summits.
Thousands of police and military personnel from around Italy have been brought in to guard a six-mile-long chain-link security fence as the area around the harbour has taken on the appearance of a ghost town.