The billionaire financier and philanthropist, George Soros, says this week's G20 summit in London is a make-or-break event as the threat of a global depression looms.
Mr Soros says the international financial system had collapsed because it was flawed and it had to be restructured.
He says the system has to be reconstructed, by countries agreeing and working together.
Mr Soros says the G20 meeting has to come up with concrete solutions to help the developing world in particular, which has been been worst hit.
He also says rebuilding means the previous economic system has to be scrapped.
Mr Soros says "I don't think we'll ever be back to where we came from. It should be recognised that the last 25 years were an aberration and we cannot go back there. We have to reconstruct the financial system from its foundations up."
He blamed regulators and the financial sector for the meltdown as, he says, they "participated in this crazy boom built on false premises on the belief that markets are self-regulating and should be left alone".
Protesters told to give governments a chance
United States Vice President Joe Biden has called for G20 protesters to give governments a chance to tackle the economic crisis.
At a G20 warm-up meeting in Chile, Mr Biden said heads of state would agree proposals to remedy the crisis at the meeting in London.
Mr Biden says hopefully the leaders can make it clear that the G20 meeting will produce some concrete proposals.
He says the success of an economic recovery is measured not just by a growth in the collective GDPs of countries, but by whether the standard of living of citizens increases directly.
Tens of thousands of people have marched through the streets of European capitals ahead of the G20 summit, to express their anger at the human cost of the financial crisis.
An alliance of groups has called for action on poverty, jobs and climate change from the leaders of the world's 20 biggest economies meeting in London on Thursday.
In Britain, trade unions, aid agencies, religious groups and environmentalists joined together under the slogan "Put People First" and police estimate 35,000 people marched through London to demand reforms.
The protest was mirrored in other big EU economies, with about 10,000 people marching in the German capital Berlin and its financial capital Frankfurt.