Mr. Obama is unlikely to push for specific commitments from other countries on stimulus spending to bolster their own economies, White House officials acknowledged Saturday in a teleconference call, despite the fact that administration officials would like to see European countries, in particular, increase their spending to try to prompt a global economic recovery.

“Nobody is asking any country to come to London to commit to do more right now,” said Michael Froman, deputy national security adviser for international economic affairs. Instead, world leaders at the meeting will try to “do whatever is necessary to restore global growth,” Mr. Froman said.

The challenges stem in part from lingering unhappiness around the world at the way the Bush administration used American power. But they have been made more intense by the sense in many capitals that the United States is no longer in any position to dictate to other nations what types of economic policies to pursue  or to impose its will more generally as it intensifies the war in Afghanistan and extracts itself from Iraq.

“There is a direct challenge under way to the paradigms that America has been trying to sell to the rest of the world,” said Eswar S. Prasad, a former China division chief at the International Monetary Fund. The American banking collapse, which precipitated the global meltdown, has led to a fundamental rethinking of the American way as a model for the rest of the world. Yet even as his presence stirs opposition to particular American policies, Mr. Obama is being welcomed by many Europeans as an embodiment of American ideals.

In Prague, where Mr. Obama will stop later in the week, local officials are installing a hot line for residents to find out about street closings. In Strasbourg, France, site of a NATO meeting, protesters are planning an “international resistance camp” with antiwar actions designed to press Mr. Obama to get American troops out of Afghanistan. In Istanbul, his last stop, workers are polishing up the Hagia Sophia basilica-cum-mosque-cum-museum for the expected visit.