"This is the day the world came together to fight back against global recession," British Prime Minister Gordon Brown said at the summit's conclusion, though he cautioned that "there are no quick fixes."
U.S. President Barack Obama hailed the summit as a "turning point in our pursuit of global economic recovery."
Mr. Obama said the G20 nations agreed on "unprecedented steps to restore growth and prevent a crisis like this from happening again."
In a communique capping a dramatic one-day gathering, the leaders announced the creation of a supervisory body to flag problems in the global financial system - but did not satisfy calls for new stimulus measures.
They did, however, bridge the gap between the United States and some European nations over how far to regulate the market and curb the excesses that sparked the global economic crisis.
Brown announced agreements to place tighter restrictions on the "shadow banking system" - pulling hedge funds into the realm of global regulation.
"The banking secrecy of the past must come to an end," Brown said.
The Group of 20 also agreed to put an end to tax havens. Brown and French President Nicolas Sarkozy said on Thursday that Organization for Economic Cooperation and Development would circulate a name-and-shame list of countries that serve as tax havens. Those nations could also face sanctions if they did not submit to new regulations.
Sarkozy, who earlier had threatened earlier to walk out if unsatisfied with the outcome, also praised Mr. Obama for helping to create consensus and persuade China to agree to publish lists of tax havens.
"There were moments of tension," Sarkozy said. "Never would we have thought to get as big an agreement."
In its pledge to increase foreign assistance, G20 leaders said the IMF would receive $500 billion to the IMF so it can increase its loans to governments struggling because of the financial crisis, Brown said. G-20 nations will also give $250 billion in trade finance over next 2 years.
Vincent Cable, one of the top economic thinkers in Great Britain and a member of Parliament, told CBS Evening News anchor Katie Couric he thought the summit was encouraging.
"They didn't, of course, reach agreement on the fundamental issue of how you stimulate the world economy but, on many other issue, there was something really positive," Cable said. "I think particularly putting a lot of money into their international monetary fund, the world bank, it will help other countries head off what otherwise could be really disastrous set of financial crisis."
Brown also said the G20 nations vowed to fight off protectionism, saying the increased trade would be essential to a global economic recovery.
"I hope that the heads of government mean what they've said about naming and shaming countries that resort to protectionism," Cable told Couric. "We cannot go down that road again."
Brown said the G20 will create a new financial stability board to ensure cooperation across frontiers, to spot risks to the world economy and - together with the IMF - provide "the early warning mechanism that this new global economy needs."
Brown says it is essential that the world does everything necessary to "rebuild trust" and make sure "a crisis such as this" never happens again.
The final deal will also include proposals making sure companies more tightly link executive salaries to performance, a measure that reflects public outrage about the huge retention payments, bonuses and golden parachutes granted to banking chiefs before the sector collapsed.
Sarkozy and Merkel had been adamant that the G20 meeting must take to more closely regulate banks, hedge funds and other financial institutions.
The push for greater regulation may signal a belief that unfettered capitalism has gotten out of control, reports CBS Evening News anchor Katie Couric.
President Obama had three tangible goals for the summit, White House Press Secretary told CBS' The Early Show Thursday. Among them were tighter financial regulations and increased help to emerging economies - two ideas largely embraced by other leaders. But Mr. Obama also wants to see other nations, particularly in Europe, step up their own stimulus programs.
Despite the absence of coordinated stimulus, 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.
While the leaders did not announce any new stimulus measures, Brown said the $1 trillion deal to boost funds for the International Monetary Fund, World Bank and other global institutions was unprecedented.
"For the first time we have a common approach to cleaning up banks around the world to restructuring of the world financial system. We have maintained our commitment to help the world's poorest," Brown said. "This is a collective action of people around the world working at their best."
The G20 leaders also said that developing nations - hard-hit and long complaining of marginalization - would have a greater say in world economic affairs. They vowed to renounce protectionism and pledged $250 billion in trade finance over the next two years - a key measure to help struggling developing countries.
European and ahead Thursday as the outcome of the summit came into view.
Meanwhile, the European Central Bank by a quarter of a percentage point to 1.25 percent Thursday - a record low - to help the 16 countries that share the euro combat the economic downturn.
As leaders met in the Docklands, a former shipping area on the Thames river that was redeveloped as an international business center, protesters began a second day of demonstrations. Security was tight at the summit venue; hundreds of police manned barriers and checkpoints around the security perimeter.
Demonstrators gathered outside the London Stock Exchange near St. Paul's Cathedral. Riot police took up positions as well, ringing the stock exchange.
French daredevil Alain Robert scaled Lloyds of London's high-rise headquarters as office workers below snapped photos. Robert, dubbed the French spider-man, has scaled dozens of tall structures around the world without ropes or harnesses to draw attention to global warming. He was later led away by police.
Near St. Paul's Cathedral in the financial district, protesters played a giant Monopoly game.
"The question is of course who has got the monopoly? It is fairly obvious the G20 are the global financial elite," said protester Clare Smith, 27. "Meanwhile the poor are getting poorer."
Some 4,000 anarchists, anti-capitalists, environmentalists and others protested Wednesday around the Bank of England for what demonstrators had called "Financial Fool's Day."
They repeatedly overwhelmed police lines, vandalizing the Bank of England and smashing windows at the Royal Bank of Scotland. An effigy of a banker was set ablaze, drawing cheers.
Police arrested over 100 people.
The diplomatic highlight of Wednesday came out of Mr. Obama's meeting with Russian President Dmitry Medvedev. The two announced an agreement to - stalled since 1997 - and Mr. Obama accepted an invitation to Moscow in July.
It was a marked change in direction for the relationship between the two nations, which declined to near-Cold War levels under former President George W. Bush, primarily due to differences on NATO's eastward expansion and U.S. plans to install missile defense hardware in Eastern European nations, close to Russia's borders.