A line of police officers blocks protesters Tuesday in Pittsburgh. The two-day Group of 20 Summit will bring nearly two dozen world leaders to the city Thursday and Friday.

PITTSBURGH — Bolstering a wobbly global economy, restraining greedy bankers and plotting a future course for sustainable growth — the leaders of the world's major economies have no shortage of items on their to-do list when they meet Thursday and Friday.

The problem is that with the global economy on the mend, they could encounter waning enthusiasm to launch bold initiatives, especially if those efforts would limit their political maneuvering room back home.

President Barack Obama is already facing the likelihood that one of his major goals will be watered down. He wants the G-20 to agree to a new global compact to avoid the dangerous imbalances that many believe played a major contributing role in pushing the world into a severe and prolonged recession that has cost millions of lost jobs and wiped out trillions of dollars in wealth.

If the economy poses some tough challenges at the conference, so does security.

Thousands of law enforcement authorities began Tuesday to erect a security perimeter around the David L. Lawrence Convention Center in downtown Pittsburgh, where the talks will be held, an effort put on an even higher alert after the discovery of an alleged bomb plot in Denver and New York set off a series of counterterrorism bulletins.

The Coast Guard was closing the city's three rivers from 6 a.m. Thursday to 10 p.m. Friday and patrolling with 11 boats. State police will have 1,200 troopers in the area, including some on helicopters and airplanes. The Pennsylvania National Guard is heading a task force of 2,500 troops and other Defense Department personnel for crowd control and other duties.

It is likely that the G-20 will endorse the U.S. call for a new "framework for sustainable and balanced growth," but without any major way to enforce commitments made to restrain imbalances such as China's massive trade surpluses and the United States' surging budget deficits.

The G-20 is expected to agree to a peer review supervised by the International Monetary Fund. However, a similar pilot project launched with fanfare by the IMF in 2006 went nowhere.

Many believe that the huge imbalances earlier this decade, including a global credit glut fueled by China's trade surpluses, played a major role in the subprime mortgage boom in the United States. The boom ended in a disastrous housing bust that led to the worst financial crisis in the United States since the 1930s.

But even without a credible enforcement mechanism, supporters of the approach contend it can serve a useful purpose as a warning mechanism for countries pursuing inappropriate policies.

"This new framework is the key to resolving issues where different continents can better work together to achieve the growth level we need," British Prime Minister Gordon Brown told reporters.

The G-20 leaders are also vowing to adopt tougher rules to keep banks from engaging in the kind of risky behavior that brought on the current crisis, but they are pushing different approaches.

German Chancellor Angela Merkel, who will face German voters Sunday, and French President Nicolas Sarkozy have sought caps on excessive bonuses paid to bankers, which they contend end up rewarding risk-taking. Sarkozy has even threatened to walk out if the language on the bonus issue isn't tough enough.

But the French president has softened his initial demands, conceding after a summit of European Union nations last week that getting agreement on mandatory caps could be a problem.

In the run-up to the summit, Merkel has expressed concerns that with the global economy showing signs of improvement and financial institutions starting to stabilize, momentum for reform could be lost.

"The window of opportunity is closing," she said, arguing that "by 2010 we must have a clear overview of what we have accomplished."

The United States is pushing a proposal to require banks to hold larger amounts of capital, the reserves they use to guard against losses. The G-20 is expected to broadly endorse this goal but leave it to financial regulators to try to hammer out the details, a goal that has not proven easy in past attempts to set global rules.

China has resisted the rebalancing plan, fearing it could be used as a club against China's huge trade surpluses.

In the end, analysts believe the G-20 leaders will find a way to paper over their differences, if for no other reason than they fear signs of discord could spook still fragile financial markets.

"The leaders realize that compared to their earlier meetings when the global financial system was in disarray, conditions are better now," said Mark Zandi, chief economist at Moody's Economy.com. "But it's a very fragile stability. There are still very large institutions on edge and big parts of the credit system are still dysfunctional."