Nonetheless, he cautioned, "There are no guarantees."
The new U.S. leader said the heads of industrial countries that met in London agreed on "unprecedented steps to restore growth and prevent a crisis like this from happening again." (.)
He spoke shortly after G-20 leadersin financing to the International Monetary Fund and other global institutions and declared a crackdown on tax havens and hedge funds.
The leaders also announced the creation of a supervisory body to flag problems in the global financial system - but did not satisfy calls from the U.S. and others for new stimulus measures.
Despite that failure, Mr. Obama called the one-day London gathering "very productive" and historic because of the scope of the challenges the world faces in righting the economic crisis that's wreaking havoc on virtually every country.
"The challenge is clear. The global economy is contracting," Mr. Obama said.
In a one-hour news conference packed with media from across the world, the president was asked about diminished esteem under his predecessor, George W. Bush.
"We're starting to see some restoration of America's standing in the world," he said.
"I do not buy into the notion that America can't lead in the world," Mr. Obama added. He also said it is "very important for us to be able to forge partnerships as opposed to dictating solutions."
In response to a question from CBS News White House Correspondent Chip Reid, the president acknowledged that some participants during the summit made comments that seemed to blame America and Wall Street for triggering the crisis that has spread around the world.
"It's hard to deny that some of the contagion did start on Wall Street," Mr. Obama said, asserting that some firms took "wild and unjustified risks" and some government regulators were "asleep at the switch."
Nonetheless, he said, there were problems in other parts of the world as well.
When he called on Reid, the president offered condolences to the correspondent on the death of his father. (Read more.)
As for the summit, he praised the G-20 nations for rejecting protectionism that hampers foreign trade and could deepen the economic crisis, and he urged global unity, saying, "We owe it to all of our citizens to act."
Still, he said, "it is hard for 20 heads of state to bridge our differences."
"I think we did OK," he said, speaking generally about his trip. "When I came here it was with the intention of listening and learning but also providing American leadership."
A day-by-day guide to one of the most closely watched presidential trips in recent memory.
He said the document the G-20 produced and actions that will follow "reflect a range of our priorities."
"We wanted to make sure we had a strong, coordinated response to growth" and "we thought it was important we had a strong, coordinated regulatory response," Obama said - and added that both were achieved.
The president also called for "transparency and accountability" when it comes to the salaries and bonuses of executives. (Read more.)
When asked, Mr. Obama could not point to an individual summit accomplishment that would help recession-battered Americans beyond general points such as fighting protectionism and making the global economy work together.
But he said the summit communique "affirms the need for all countries to take fiscal responses that increase demand, that encourages the openness of markets. Those are all going to be helpful in us being able to fix what ails the economy back home."
"This is not a panacea, but it is a critical step," Mr. Obama said.
He declined to specify where the White House compromised, saying the final communique reflected a consensus of world leaders.
"Each country has its own quirks or issues that a leader may decide was really really important," Obama said.
He said he was committed to "forging a consensus instead of dictating our terms" and argued that the United States acting alone would only be "halfway effective, not even half."
At one point, in response to a question from a Chinese journalist, the president quipped, "I'm terrible with those little catchphrases and sound bites." (Read more.)
On his whirlwind, weeklong European trip, the new U.S. president also met privately with other heads of state on the sidelines of the summit.
He said he had had productive one-on-one talks with leaders of Russia, China, Great Britain and India, on topics that included ways to "reduce the nuclear threat" throughout the world. He called for a coordinated response to North Korea's plans to launch a multistage rocket in the coming days.
Earlier Thursday, Mr. Obama and South Korean President Lee Myung-bak agreed on a need "for a stern, united response from the international community" in light of North Korea's efforts toward a threatened satellite launch, and the White House announced that Lee would visit Mr. Obama in Washington on June 16. (.)
Officials said the two leaders also discussed a free trade pact between the countries that would slash tariffs and other barriers to trade.
Mr. Obama has hinted he might seek to renegotiate a 2007 deal between South Korea and the U.S. that was hammered out under former President Bush but that legislators in the two countries failed to ratify amid opposition from farmers and labor groups. Officials said Mr. Obama told Lee that he understood there were difficulties with the deal on both sides, but that he wanted to "make progress" on it.
Mr. Obama also met on Thursday with Indian Prime Minister Manmohan Singh to discuss how their countries could work together. Obama had been expected to reassure Singh about plans to boost aid to India's rival, Pakistan.
The president wouldn't declare that the steps the G-20 took would prevent a deeper recession.
"In life there are no guarantees; in economics there are no guarantees," Mr. Obama said, coughing and sniffling at times as he battled a cold he said he'd been fighting all week.
"There are always risks involved," he added. Still, he said, "I have no doubt, though, that the steps that have been taken are critical to preventing us sliding into a depression."
He called the G-20 prescriptions bolder and likely to work more quickly than any international response to an economic crisis in memory, and he said he was confident the steps would have "a concrete effect" in each nation of creating jobs, saving jobs, expanding the economy, loosening credit and restoring confidence in the financial markets.
The steps "were necessary," Mr. Obama said. "Whether they were sufficient, we've got to wait and see."