"It was historic because of the size and the scope of the challenges that we face and because of the timeliness and the magnitude of our response," he said.
Prime Minister Gordon Brown said there was "no quick fix" for the world economy but there was a commitment to do whatever was necessary.
"This is the day that the world came together to fight back against the global recession, not with words, but with a plan for global recovery and for reform and with a clear timetable for its delivery," Mr Brown said.
Another G20 meeting will be held in New York in September to check on progress, the BBC has learned.
The deal was announced shortly before the European stock markets closed and gave leading indexes a significant boost.
London's FTSE 100 index of leading shares ended 4.3% higher. In Paris, the Cac 40 jumped 5.4% and in Frankfurt, the Dax rose 6%.
On behalf of the G20, Mr Brown announced the following steps:
Bankers' pay and bonuses will be subject to stricter controls
Gordon Brown announces the G20 deal
A new Financial Stability Board will be set up to work with the IMF to ensure co-operation across borders and provide an early warning mechanism for the financial system
There will be greater regulation of hedge funds and credit ratings agencies
A common approach to cleaning up banks' toxic assets has been agreed
The world's poorest countries will receive $100bn extra aid
G20 countries are already implementing the biggest economic stimulus "the world has ever seen" - an injection of $5tn by the end of next year.
The IMF has been one of the biggest beneficiaries of the G20 summit.
The resources it has to help troubled economies will be increased to $500bn.
An overdraft facility will also be increased to $250bn (in the IMF's currency, so-called Special Drawing Rights) that all members can call upon.
Mr Brown said that there would be a crackdown on tax havens to prevent the loss of sorely needed tax revenue.
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