Leaders of 20 of the world's leading and emerging economies are gathered in Pittsburgh, seeking ways to fight the recession, and keep economic troubles from breaking out again.
Friday, presidents, prime ministers and other heads of state are scheduled for working sessions focused on the economic issues that pushed the world into recession.
They hope to change rules and customs to avoid another such meltdown in the future.
Before the summit began there was considerable public debate over bonuses paid to bankers, and efforts to limit them. French and German officials say the bonus system encouraged reckless risks that threatened the financial system. The Europeans want new rules to limit such payments.
U.S. officials pressed for rules requiring banks to keep larger reserves to cope with bad loans.
U.S. Treasury Secretary Timothy Geithner says the leaders are close to consensus, and there is a need for swift action.
"We don't want to see these reforms take effect two years from now. We don't want to see them take effect next year," he said. "We want to see them take effect now so they affect compensation process today, not tomorrow."
Geithner says there are many signs the economy is recovering, but he says there is still a need for major changes by many nations, including the United States.
"For too long, Americans were buying too much, and saving too little … that is no longer an option for us or for the rest of the world," Geithner said.
Geithner argues that major exporting nations can no longer look to the United States as an engine that will pull their economies out of recession. He says the U.S. consumer has cut spending. That may mean exporters will have to boost demand in their own nations to keep their economies growing.
Leaders are also discussing when and how they will cut back efforts to stimulate their economies, and expanding the role of emerging nations in economic decisions affecting the world.
As the leaders arrived in Pittsburgh, protestors gathered, urging more efforts to fight global warming, and accusing the G20 of doing too little to help the poor.
The demonstrators were kept away from the summit site by thousands of police, troops, helicopters, boats, tall fences and occasionally, tear gas.