This was a week of lost opportunity for the world's financial establishment.

It may be true that there is a certain artificiality about the annual meetings of the World Bank and International Monetary Fund: they are planned at least a year in advance with an agenda that is cut and dried. The thousands of delegates and hangers-on who pour into a big city like this for the joint sessions are part of a mob scene, which is hardly suitable for making big decisions.

Yet the public perception that this is a gathering of the world's financial elite is not totally incorrect. The big bankers and their counterparts in government and in the international lending agencies are here, and they seem to have missed a chance to shore up confidence in a fragile economic system.

The news, not entirely unexpected, is that they did virtually nothing. By the time the sessions opened formally on Monday morning with some eloquent words from Prime Minister Pierre Trudeau of Canada to the effect that unemployment, inflation, Mexico's travails and some other problems "are generating a fear in the minds of some of our people," this particular ball game was over. The fact that Trudeau dared to draw some cautious parallels with the big Depression didn't matter.

To the extent that there is business done at these meetings, it transpires on the weekend, when the policy-making boards of the IMF and the World Bank meet privately.

But these groups were stymied, largely because the Reagan administration fears that giving the IMF and the World Bank too much money will only produce an inflationary binge managed by international bureaucrats.

The IMF staff, headed by a French aristocrat named Jacques deLarosiere, has for the past year been trying to get agreement that its regular capital be doubled from $67 billion to $125 billion. It would happily have settled for a two-thirds increase, to $110 billion.

But the American team for this purpose, headed by Treasury Secretary Donald T. Regan, does not believe that deLarosiere could even hand out that much money unless the IMF were to become yet another aid agency, concentrating on "development." And in some quarters of the Reagan administration, "development" is a dirty word that translates into welfare programs.

The Reagan administration pays lip service to the World Bank as preferable to the somewhat kookier New International Economic Order dreamed up by United Nations radicals for helping the Third World. But as Regan made plain in a speech (that one delegate said could have been made 20 years ago), the United States really thinks the World Bank is inefficient and that the "private sector could do a better job."

Secretary Regan is quite right when he says that if Mexico is in a mess despite its huge oil and other natural resources, one reason is that Mexico, like many another backward nations, has been tempted to spend beyond its means. But as DeLarosiere made plain, it has been encouraged to do so by some of the biggest American and European banks.

Almost literally, bankers have scooped money out of their vaults and pressed it on poor nations in Latin America, Asia and Eastern Europe. So long as they can get some of their interest paid back -- never mind the principal -- they traditionally have been willing to "reschedule" the debt.

DeLarosiere said that commercial banks now have loans to the poor countries that do not have oil resources of their own amounting to about $325 billion. But no one really knows how much there is. One responsible official believes that the total amount of commercial-bank loans to governments all over the world amounts to more like $750 billion. And no one doubts the rumors that some banks have a huge percentage of their total capital lent out to Mexico.

The disappointing thing is that the meeting in Toronto failed to follow up on a rather perceptive and quick response to the Mexican situation. The United States, the IMF, central banks in Europe and the major multinational banks involved in the Mexican crisis got together quickly and are in the midst of developing a program that may offer Mexico a way out of its crisis.

It is a pity that the meeting here did not follow up with a substantial boost in the IMF long-term resources, along with some emergency funds that could be used in the short term. For this failure, the Reagan administration has to take virtually the entire blame. It could have taken an important initiative to stave off a world financial crisis at no cost to the American budget deficit.