The Prime Minister said he believed decisions made at the gathering would set the world back on a path to sustainable growth.
Under the deal, the G20 will become the “premier” body for monitoring international economic problems, Mr Brown said.
New rules for remuneration in the financial services will be “tougher” than many had expected, ensuring there is “no return to the bad old days”.
The nations will also agree to maintain their fiscal stimulus, which has saved about 10 million jobs over the past year, according to Mr Brown.
“The old systems of economic co-operation are over,” Mr Brown said. “New systems of economic co-operation, from today, have begun.”
Mr Brown said he will urge fellow leaders at today’s summit to push ahead with the multibillion-pound packages of fiscal and monetary stimulus which they have activated to beat the recession.
Moving to an “exit strategy” in which measures such as state spending, low interest rates and tax cuts are withdrawn would risk bringing the global economy’s fragile recovery to a halt, he warned.
Mr Brown cautioned: “The recession is not over. It is not automatic that we are going to recover. We have got to ensure that that recovery happens.
“The path to recovery is still very fragile - fragile in the banking system and fragile in the way that different economies are finding different difficulties as they move forward.”
The Prime Minister said Britain’s Conservatives were isolated internationally in their insistence that state support for the economy should be scaled back immediately, which he said would put growth and jobs at risk.
Mr Brown said: "A new mechanism has been created at the G20. It has got a new process that Britain and America have been working together to suggest.
"That process, I think, has got more chance of delivering results than any of the previous attempts that have been made since the Second World War.
"We have had international economic institutions, G7s , G5s and G8s. But this is the first time we are attempting to bring together all the major economies of the world and we are attempting from today to set in place a plan for sustainable growth with a framework and a compact between countries for making that framework work."
Mr Brown indicated that one way of injecting further momentum into the world economy might be to draw on the reserves, totalling around 7 trillion US dollars, held by countries around the globe, which he suggested might be larger than was needed to protect states against instability.
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