Standing alone at the beginning of a gloomy conference of the World Bank and International Monetary Fund here today, representatives of the Reagan administration resisted sober warnings from all sides that poor countries need the promise of a great infusion of aid to tide them over the world's worst economic crisis in nearly 40 years.

"These hard times, and these perceptions of looming crises," Canadian Prime Minister Pierre Elliott Trudeau said in his welcoming speech to delegates from 144 nations, "are generating fear in the minds of some of our people" that he compared to the mood of the troubled 1930s.

Trudeau called on the IMF to "take the lead as a matter of urgency" to help both nations and private institutions to avoid collapse.

But the IMF, which had hoped to boost its normal lendable resources from $67 billion to about $110 billion, was rebuffed by its biggest shareholder, the United States.

Alone among leading nations, the United States took a stand against an unspecified "substantial increase" in IMF resources at Saturday's meeting of the IMF's policy-making Interim Committee.

Asked today by reporters to explain why the United States finds itself at odds with other industrial nations, even such a close ally as Great Britain, Treasury Secretary Donald T. Regan said, "In previous times of prosperity in the United States, we could afford to be generous. At the moment, the nations of the world have been expressing alarm at the size of the U.S. deficit. We ourselves are aware that our deficit is larger than ever, maybe not as a percentage of GNP, but at least in dollars.

"We have to cut back on that. . . . Naturally, the rest of the world is dismayed to see the United States cutting back. But, nonetheless, that's reality as we face it.

"We can't do everything: we can't be a defender of the Western world, as we know we are, and continue to have high defense expenditures, and at the same time run the same social programs that we have been running in the U.S., and to give as much financial aid abroad as we have," Regan said.

"We give as much as we can, and other nations would like more. And as a result they express their disappointment."

Regan said the United States remains the largest contributor to the World Bank and the IMF and that "other nations must recognize that the United States has its limitations."

He also said the Reagan administration has a "more upbeat" assessment of prospects for both the U.S. economy and the global economy than anyone else. Regan estimated real growth in the industrial world next year at 3 to 4 percent.

World Bank President A. W. Clausen, who told delegates that "we are meeting at a time when the economic situation for most of our member countries -- developed and developing alike -- is grim," estimated less than 1 percent growth. IMF managing director Jacques deLarosiere said there would be no growth in 1982 at all.

In a sober assessment of current "strains" on the world's monetary system, deLarosiere said it is in a transition period in which "hardships will be felt both domestically and internationally."

But he offered the hope that the world would muddle through the crisis provided that the international financial community "maintains a responsible attitude by supporting those borrowers" who are trying to make the necessary adjustments to reality.

Nonetheless, as Trudeau suggested, the meeting here is filled with anxiety as a result of Mexico's problems in repaying its foreign debt, which has triggered a flood of rumors about other Latin American countries.

Some of this anxiety might have been assuaged if the IMF's Interim Committee had been able to announce an intention to boost its lending quotas by at least two-thirds.

Although such an increase would not have taken effect until 1985, former assistant treasury secretary Fred Bergsten said it would have been received as evidence that the rich nations were willing to make a long-range commitment to problems certain to last throughout the decade.

Trudeau referred to the Depression of the 1930s by observing that "those who toiled" on the Bretton Woods agreement setting up the IMF and World Bank in 1944 "were driven by the intellectual conviction that the economic and financial chaos of the 1930s need not have happened . . . .

"Today we must approach our difficulties with the same faith and confidence in our ability to bring stability and prosperity to our international economic community."

DeLarosiere's speech was an unusually blunt and comprehensive review of the plight of debter nations that he said were caught up in the crosscurrent of high interest rates and recession.

He warned that commercial banks have so expanded their role in international lending that they are now "dominant" in financing these nation's balance of payments deficits -- a function that used to be almost exclusively in the province of governments.

He reported that by the end of 1981 the poor nations had racked up the astonishing total of $540 billion in foreign debt, of which $440 billion was long-term debt over three years. He said about 60 percent of the $540 billion total -- or $325 billion -- is owed to private financial institutions.

This large increase in commercial debt, coupled with soaring interest rates and the recession in the industrial world, is a primary cause of the current trouble. The total debt service owed by these countries this year will amount to almost one-fourth of all the money that they earn from their exports.

Since interest rates account for so much of these debt costs, one small silver lining is the recent sharp fall in interest rates, if it continues.

But deLarosiere advised banks that they carry the responsibility for preventing the crisis from spreading. "Bank lending that gives a country an unwarranted sense of financial ease is not sustainable over the medium term," he said.

"When financing only has the effect of allowing a country to live beyond its means by, for example, supporting excessive consumption, it serves neither the interests of the borrower nor that of the financial community."