For at least a couple of years, the International Monetary Fund has been trying to shed its "Uncle Scrooge" image in the Third World; whether deserved or not, its reputation as a tough lender, demanding austerity conditions from the poor nations, is widespread.

Now, as the IMF prepares to convene in Washington Tuesday through Friday in its 36th Annual Meeting with its sister Bretton Woods agency, the World Bank, it is being accused by the United States of going too far in the other direction. This turn of events will give the financial leaders of 143 nations plenty to chew over during preliminaries today and tomorrow at the Sheraton Park Hotel.

"We think that the IMF could be a little more strict . . . with some of the larger developing countries and some of the more prominent less-developed countries," U.S. Treasury Secretary Donald T. Regan -- also the U.S. governor for the fund and the bank -- told reporters.

Nor did the World Bank escape American attention. Long the target of arch-conservatives in the Reagan administration who see it as an instrument of "socialism" because it deals government-to-governnment, the bank was told by Regan in no uncertain terms to rely heavily on the private sector.

The meetings, which may again witness an effort of the Palestine Liberation Organization to be admitted as an observer, will be chaired by Uruguay's minister of economy and finance, Valentin Arismendi Elgue. Preliminaries have been under way for a few days with meetings of the Group of 24 -- a Third World group -- and the IMF's policy-making Interim Committee, which began its deliberations yesterday and scheduled a press conference today.

The Joint Bank-Fund Development Committee, chaired by David Ibarra Munoz, Mexico's secretary of finance, begins a two-day session today to review Third World prospects. At some time before the annual meetings start, the Group of Ten major industrial nations' finance ministers will also caucus, as will the Big Five among the Ten -- the United States, West Germany, Japan, France, and Great Britain.

The probable collision of U.S. policy with bank-IMF practices and programs may tend -- at least momentarily -- to drive the world's macro-economic difficulties into the background at the annual meetings. But the outlook in the United States, Europe, and the Third World -- indeed, almost everywhere except in Japan -- is bleak enough to assure the usual batch of grim speeches by ministers, as well as a sober communique' from the Interim Committee meeting today and from the Development Committee tomorrow.

The new American policy, certain to put a greater credit squeeze on the Third World, is justified by the Reagan administration as a consequence of budget austerity being experienced in the United States. But it also reflects the ideological disposition of the most right-wing elements among Reagan's advisers who regard development-aid programs -- especially the bank's -- as one big welfare program.

To some extent, Secretary of State Alexander Haig has tried to maintain a more generous stance toward the Third World countries than Treasury Secretary Regan, at least in his public rhetoric. But the differences are mostly a matter of style. Although State Department officials are more favorably disposed to the multilateral lending principle than their opposite numbers at Treasury, a key State Department policymaker in the area of economic issues nonetheless observes that "the degree of government involvement is not a key to the development process." He adds that the biggest benefit the industrial nations can devise for the poor is not aid programs, but getting stronger themselves so that the poor nations can export more. Another version of trickle-down?

What it comes down to, whether one hears it from the Treasury or State Department, is a deep suspicion of Third World ideologues and Brandt Commissionites who talk in terms of a "new international economic order," which is regarded in Reagan's Washington as double-speak for redistribution of income from the rich to the poor. The purest of the pure at Treasury would tend to lump in the bank and IMF as part of the whole Third World structure. They advocate a bootstraps-up operation in the less-developed world, replete with tax reductions and incentives for business.

"The United States is bent on a new supply-side imperialism," says a bitter Democratic congressional aide. "There is a view in the administration that there is too much power lodged in governments, and this is sending a worrisome message to the Third World." In many situations, it is pointed out, the Reagan administration's desire to give aid more directly to the private sector, and less through governments, will be frustrated because in some of the poorest nations there is no private sector. Regan, a bit less ideological than his own staff, concedes the latter point.

But Ronald Reagan is convinced that supply-side economics -- read that Reaganomics -- should have a chance in the rest of the world, as well as in the United States. But he won't have an easy time convincing the other rich powers, and certainly not the poor ones, that they can pull themselves up by their bootstraps, relying on private enterprise, just because the United States did it a century ago. The miracle quotient of Reaganomics is just as untested abroad as it is at home.

For the IMF, the new, hardened attitude of the United States -- for which the Americans claim support by all "major trading partners," including socialist France -- is a bitter pill to swallow. Until now, the U.S. ideological ire has seemed directed more toward the bank, and the IMF seemed to have U.S. blessing in seeking to build up its resources and to make loans of somewhat greater maturity, recognizing that the economic plight of most poor nations can't be resolved overnight.

At the annual meetings, the IMF will be hearing a different pitch from the United States. "We will be telling them that our representatives will be watching their lending policies , and insisting upon more strict fiscal policies and more strict monetary policies within the countries themselves, as conditions for the loans ," Regan said. Publicly, the major differences in approach are being covered up with the assertion that the IMF itself in the past couple of months has turned tougher in the "conditionaliy" it demands for loans, and that the Americans merely want the fund to maintain its present stance.

In fact, there has been an undercurrent of irritation between the IMF and the United States ever since the Interim Committee, at its spring meeting in Gabon, rejected the U.S. choice of British Chancellor of the Exchequer Geoffrey Howe as chairman, and turned instead to Canadian Finance Minister Allan MacEachan. The United States concluded that IMF Managing Director Jacques de Larosiere had supported MacEachan, supposedly more in tune than Howe with Third World demands. For its part, the IMF management was not used to the rather direct and blunt approach of two influential Treasury undersecretaries, Beryl Sprinkel and Tim McNamar.

At the IMF there is a suspicion that the frontal U.S. attack is designed to make three things clear: First, that the United States intends to hang tough in its resistance to a new issue of Special Drawing Rights, the paper money created by the IMF. Second, the United States wants to lower expectations of the Third World nations -- all of whom will be at the annual meetings here -- that the U.S. will come forward with any new big kind of program at the Cancun North-South summit on Oct. 22 and 23.

At the Ottawa summit, American officials had voiced the fear that "global negotiations," as demanded by the Third World, would be accompanied by a proposal for what the United States would regard as a massive and inflationary issue of SDRs.To be convinced of the wisdom of a new issue of SDRs, Regan said this week, the United States would have "to see inflation abating throughout the world."

And third, the United States appears to be making an effort to put the brakes on a scheduled $5.5 billion loan to India, the biggest in IMF's history -- which according to IMF insiders is being made "without agonizing conditions." For the United States, the IMF loan to India (which over the years has been IDA's biggest client) appears to epitomize all of its complaints about the unecessary generosity of both the World Bank and the IMF.

"I would merely point out to you," Regan told reporters, "that some of the greatest recipients of aid from IDA in recent years still have major problems that the loans certainly don't seem to have cured, and we're wondering why."

The ability of some of the larger and more advanced of the developing nations to tap the resources of the IMF and World Bank at a time when it is difficult to get the Congress to come up with aid money clearly irritates the United States. Oil-rich Mexico was the largest borrower last year from the World Bank, and the U.S. government asks why Mexico shouldn't be required to rely more on the private market. At the same time, it thinks some other nations should be "graduated" -- that is, from the soft-dollar window to the hard-dollar window of the lending agencies.

"Does a country always stay as a less-developed country and therefore eligible for IDA loans , or should they finally, like children, be asked to leave home, and set up -- you know -- on their own a little bit more?" Regan asked. These and related issues are being examined in a Treasury review of the U.S. policy toward all of the multilateral agencies, scheduled to be published in a few weeks. But apparently authentic leaked versions indicate that the United States will sharply curtail or end its contributions to IDA, and will try to bring the bank and fund completely into line with Reagan's view of the world.

Secretary Regan stopped short of saying that the United States would oppose the presently planned IMF loan to India. But he crisply suggested that the IMF was in danger of becoming a concessional loan agency like IDA. Then he added:

"I'm just suggesting that there are a lot of questions about the loan to India . There are a lot of questions about it that we'll have to ask, because if we don't, they will be asked for us by our friends on the Hill . . .

As things stand, Congress is even less willing to play ball with the international lending institutions than is the Reagan administration. Bank officials say that at the annual meeting this week, other IDA contributors, seeing that the IDA-6 replenishment has unilaterally been stretched by the U.S. into a four-year instead of a three-year program, will have to decide whether to march along with the United States, or whether to try to start an IDA-7 program simultaneously with the last year of IDA-6.

There is a growing question, given the recalcitrance of Congress and the attitude of the Reagan administration, whether there will ever be an IDA-7.

"There was a time," said a discouraged bank source after Regan's press conference last week, "when we talked of an IDA-7 of $16 billion following the $12 billion IDA-6. Anything of that size seems dead as a doornail right now."

World Bank President A. W. (Tom) Clausen, very much a man of the private sector, is basically sympathetic with the administration's pitch for working hand-in-hand with the commercial banking system, especially for co-financing of projects. But he's trying to convince the Reagan administration that the private sector can't do it all, nor can bilateral aid be a total substitute for the nonpoliticized benefits of multilateral aid.

Clausen also is trying to increase the bank's leverage -- "getting a bigger bang for a buck." There is, first of all, a general capital increase soon to take effect (another $40 billion over the next four years from most countries, six years from the U.S. under its stretch-out). But Clausen -- with U.S. acquiescence -- wants to liberalize the bank's antiquated "gearing ratio," which now limits loans, dollar for dollar, to the amount of capital. This is needlessly conservative, as almost everyone now recognizes.

Meanwhile, the backdrop for this year's annual meetings is a familiar one: weak and deteriorating world economic conditions. Europe is in a recession, and frustrated by high interest rates. The United States clearly is slipping into at least a mild downturn. In its mid-year report, the IMF called the overall outlook "grim," and challenged the more optimistic tone struck by the Reagan administration. The way IMF and bank staff economists see it, inflation and unemployment problems will dominate both the industrialized and the less-developed regions for the next couple of years, which automatically places increasing trade protection at the very top of the worry list.