London ? Desperate but divided on ways to lift their nations from economic misery, world leaders converged for an emergency summit Tuesday holding scant hopes of finding a magic-bullet solution for the crisis that brought them hurrying to London.

Even as President Barack Obama and the others were arriving, the U.S. acknowledged its allies would not go along with a massive burst of stimulus spending, while Europe was forced to backpedal from hopes for tighter financial regulation.

Instead, leaders are trumpeting the limited common ground they could reach, including more money for the International Monetary Fund and closer scrutiny of hedge funds and tax havens. As for the broader issues, they’re hoping for the best — or at least that they will do no harm.

With turbulent world markets watching closely, the stakes are high, especially for America’s new president, stepping onto the world stage for the first time to deal with the economic crisis and to meet face-to-face with many other leaders.

One global change is being acknowledged: The forum for grappling with world economic problems has grown beyond the established eight post-war economies that dominated previous economic summits — the U.S., Britain, Germany France, Japan, Canada, Italy and Russia. Now, 20 nations are coming together in London, with fast-growing developing economies such as China, India, Brazil and Saudi Arabia — important players in any effort to coordinate economic policy — sitting as full negotiating partners.

“For the first time, there’s a recognition that major emerging markets and developing countries have a critical role at the table,” said Mike Froman, a White House international economic adviser.

But will that mean action to stop a global downward spiral?

Froman acknowledged that there have been few examples of international gains in times of crisis. “The depression was made ‘great’ by the lack of cooperation,” he said, noting that nations like to keep control over their own fiscal and monetary policies.

And global leaders were quarreling up to the last minute before the summit.

Adding to the pressure, French President Nicolas Sarkozy said Tuesday the leaders cannot afford to let the week pass without making substantial progress in fixing the world’s economy.

“We have to obtain results, there is no choice, the crisis is too serious to allow us to have a summit for nothing,” he said.

European countries are pushing for a tougher regulatory system for global finance, while the U.S. is emphasizing more spending — an idea that holds little interest for Europeans wary about debt.

Obama planned a round of meetings with leaders today, including Queen Elizabeth II, summit host British Prime Minister Gordon Brown and the presidents of Russia and China.

The world economy is in far worse shape than when the group of rich and developing countries last met in November and set lofty goals for international cooperation.

Trade is deteriorating, protectionism is on the march and joblessness is rising. Street demonstrations have increased, and protests are expected in London this week.