In his first official appearance as the new president of the World Bank, A. W. (Tom) Clausen yesterday took sharp issue with charges by the Reagan administration that the bank ignores the private sector and should toughen up its requirements for loans to middle-income countries, as well as to the very poorest.

In his first address to a Bank-International Monetary Fund annual meeting, he warned bluntly that to scrap or limit subsidized aid through the World Bank's concessional arm, the International Development Association, might lead to "political and social instability" in the Third World, even touching off "irrational violence" among jobless young people in poor urban slums.

"There is no alternative but having an IDA-7 the next scheduled three-year replenishment of IDA funds . Put that one at rest," Clausen told a press conference. "There is just no other alternative for the poorest nations in this world that . . . do not have access to the commercial markets."

But he said plainly that an IDA-7 would have to be structured differently, probably with less grant money from the rich nations. He implied that after 1983, IDA might also have to borrow money, from commercial markets as well as from donor countries, and that the U.S. share could be less than its 27 percent involvement in IDA-6.

The most critical of American officials have suggested that the bank's programs lately have degenerated into the equivalent of worldwide welfare. Others have raised the specter that the bank has become "socialist" in its approach, because it deals on a government-to-government basis.

Clausen brushed such comments aside as irrelevant. "You can't solve all of the problems with the private sector; no one's saying that you can," Clausen said at a press conference. "You can't solve all of the problems bilateral, and say to heck with the multilateral. What's needed is a combination of all those forces."

He added he was "pleased to find the bank in even sounder shape than I had anticipated." For the decade ahead, he said the bank and IDA would "continue to . . . constitute a unique source of development capital" for the Third World, with "the focus kept on the poorest of the poor."

His defense of IDA, which has come in for the brunt of the American criticism, was in unusually strong terms. "There is a general tendency to think that soft-loans are made by soft-headed people that have got soft requirements," the former chief executive of the Bank of America told reporters. "Actually, the same criteria for economic return on the credits (as on World Bank loans) is expected."

Clausen told the annual meeting that IDA "must be continued" after the current funding expires in 1984 -- a direct response to U.S. Treasury statements last week leaving open the question whether the United States would participate in an IDA-7.

Clausen said that the United States, as "the strongest nation" in the world, selling 30 percent to 35 percent of its exports to the less-developed nations, should continue to support IDA as a matter of "self-interest."

"The issue is not whether IDA is effective," Clausen said in his speech. "It is. Nor is the issue whether IDA is a philanthropic society. It is not. An IDA credit is not a welfare check. It is a productive investment. Consider the countries that were IDA recipients only a few years ago: Korea, the Philippines, Thailand, Ivory Coast, and some 15 similar cases.

"These countries have not only graduated from low-income to middle-income economies, but today are vigorous and valuable trading partners with the developed nations."

That was a rebuttal to Treasury Secretary Donald T. Regan, who last week said that some countries now on the IDA roll should be asked to borrow at the nonsubsidized rates at the World Bank itself, and that some of the bank's customers should go out into the private market. Clausen, in effect, was responding that the bank had had such a policy in force since its beginnings 36 years ago. He said at the press conference that countries such as Spain, Greece, Israel and Ireland had been "graduated" from the bank's "hard-loan" window to the private market.

But the United States, which is readying a report on multilateral banks that will deal with this and other issues, apparently wants the bank to accelerate the "graduation" process, both from IDA to the bank, and from the bank into the private sector. Clausen acknowledged the pressure, noting that the industrial nations are being forced to keep their budgets in check in response to inflationary pressures.

In answer to press conference questions, Clausen said that a change in the bank's gearing ratio, as proposed last year by former president Robert S. McNamara is "appropriate," but not realistic for at least another five or 10 years.

The gearing ratio limits bank loans, dollar for dollar, to the amount of total capital, a very conservative ceiling in force since the origins of the bank in the post World War II reconstruction period. Clausen implied that a relaxation of the gearing ratio now might prove unsettling to the present holders of nearly $30 billions in World Bank bonds.

An easier way to expand the bank's lending ability, Clausen said, could be achieved through larger capital subscriptions, encouraged by reducing the amount of paid-in capital, presently 10 percent, to 7 1/2 percent or even less. He would also like to see increased cofinancing of projects with commercial banks.

Clausen also floated a trial balloon for the establishment of an international "framework that could provide the machinery for negotiating away the narrowly nationalistic policies discouraging international investment." His idea is to protect both the poor countries -- now suspicious of private investment -- and investing companies by basic rules on expatriation of profits, reinvestment of a share of profits, and joint ownership.