Protesters have been assembling in Pittsburgh
World leaders are meeting in the US city of Pittsburgh as the two-day G20 summit gets under way.
Economic stability, financial regulation, climate change and bankers' bonuses are set to top the agenda.
With many major economies beginning to climb out of recession - attention will turn to when and how to withdraw government stimulus packages.
Meanwhile, reports said riot police had fired tear gas at protesters on a march towards the G20 summit venue.
Thousands of extra police are on duty, as officials prepare for demonstrations by activists.
The previous G20 meeting, in London in April, was marred by clashes between police and protesters.
The leaders are gathering after the UN general assembly in New York, and are due to meet for a working dinner on Thursday evening.
US Treasury Secretary Timothy Geithner said that the world still had some way to go to emerge from the severe recession - but said he was seeing encouraging signs of a global economic rebound.
A spokesman for the White House said that financial regulatory reform was the most important agenda item for summit, but that addressing global economic imbalances was also a priority.
President Barack Obama has led a campaign to smooth out imbalances in the flow of global capital to try to secure greater long-term economic stability.
WHAT IS THE G20?
Set up after the Asian financial crisis in 1999 as a forum for finance ministers and central bankers
First G20 leaders summit in 2008 to discuss response to economic crisis
Twenty official members joined by Spain and the Netherlands and representitives of the International Monetary Fund and the World Trade Organisation
The US proposal calls on economies such as China, Brazil and India to boost domestic consumption in order to lower their trade surpluses.
Meanwhile the US and Europe would encourage more saving to reduce long-term budget deficits.
Director of the US president's National Economic Council, Larry Summers, said that a "balancing global growth approach" of said that there would have to be changes in
"The US can't should not and won't continue to experience the consumption-led growth driving very high volumes of imports and lending impulse to the rest of the world economy. That's not a sustainable financial situation for the US and that's why we're in the process of adjusting."
Cracking down on bankers' bonuses has popular appeal with the public, and so it is expected that an agreement will be reached on how that might be achieved.
There will also be talks on "regulatory harmonisation" - making sure that countries do not try to attract investment by offering looser rules.
Earlier, Chancellor of the Exchequer Alistair Darling warned bankers that the "party was over" and they must realise that the world has changed.
He wants a limit on bonuses and rules to allow banks to be able to get them back if bankers make losses later.
And he said there was a limit to how much could be achieved by regulation and that bankers must realise that they have to change their behaviour.
Mr Darling wants to use regulations to force banks to limit the proportion of their profits that they can give out in bonuses and make sure there are no rewards for failure.
Other discussions will involve the continuation of talks over whether countries such as China, India and Brazil should have greater say on the board of the International Monetary Fund (IMF).
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