United Nations, N.Y. — An iron fist in a velvet glove.
This is how many diplomats here describe Javier Perez de Cuellar, the Peruvian who is the new secretary-general of the United Nations.
In his first interview with a reporter, given Thursday, Mr. Perez de Cuellar disclosed that he plans to:
* Put the UN house in order by making its machinery cost-efficient, lean but productive, and hard-driven.
* Speak out forcefully and act tenaciously behind the scenes to try to defuse the present East-West and North-South tensions.
He also wants to ''give the UN a new sense of self-esteem, of direction, and thus to lend it a new thrust as the protector of civilized behavior in the jungle of international affairs.''
Indeed the new secretary-general, who has been in office one week, is carrying out a major shake-up in the higher echelons of the organization.
An audacious change he is expected to make this week is replacement of the director general for development and international economic cooperation, Ghana's Kenneth Dadzie, with France's progressive-minded but pragmatic Jean Ripert. The change in that position - the second-highest at the UN - and other personnel shifts are deemed essential by Perez de Cuellar to restore the UN's credibility and to help it ''to be taken seriously.''
''Cautious and bold. Independent and realistic. Quiet but firm.'' This is how he sees himself in the role of secretary-general, as he told the Monitor.
In appointing Jean Ripert, the new secretary-general is understood to want to erase the impression that has been created in the West that the entire UN system has become captive of the developing nations. He himself comes from the developing nation of Peru. The heads of UNESCO, the Food and Agriculture Organization, United Nations Industrial Development Organization, United Nations Council on Trade and Development, and International Fund for Agricultural Development all come from the third world.
If there is to be a meaningful North-South dialogue, the industrial nations must not have the feeling that the deck is stacked against them at the bargaining table.
As a member of the staff of Jean Monnet (the Common Market's founder) in the '50s, and as a former French high commissioner for economic planning, Mr. Ripert has credentials to speak for the industrial nations.
But topmost on Mr. Perez de Cuellar's list of priorities is his intention to ''streamline the UN organization, reduce its waste, put an end to certain abuses and laxity'' and improve relations between himself and the UN staff.
The new secretary-general knows his honeymoon with the superpowers and geographical groups of nations may not last long and that he will be submitted to strong pressures.
''I don't intend, however, to be anyone's puppet, nor to act as a bull in a china shop,'' he says. ''I am fully aware of the fact that I am not some kind of president of the world and that the UN is at the service of its member states.
''Some of those who clamor for moral leadership are the first to shout 'Don't meddle in my affairs' when the UN objects to some of their own misdeeds. By resorting to persuasion, by exerting moral pressure, by using diplomatic imagination, I hope to be able to contribute to bring about some movement on such delicate problems as Afghanistan, Cambodia, Cyprus, Namibia, the Middle East, to name but a few which involve the UN.''
The new secretary-general intends to devote much energy to try and stir ''global negotiations.'' He sees a need to reach an accommodation between the industrial and developing nations as the single most important contemporary problem.
On Central America and Poland, he says that ''as things stand now from a political point of view we can only take them to be internal crises.'' However, he says, the UN can examine human-rights violations that reportedly are occurring in these countries.
He also believes that a Reagan-Brezhnev meeting would help ease East-West tensions. ''Jaw-jaw is better than shoot-shoot'' he say3, paraphrasing Winston Churchill.
''I did not seek this job,'' he says. ''I was selected for it unanimously by the (Security) Council and by acclamation by the General Assembly. This gives me . . . a certain moral independence of which I intend to make use.
''By giving the example at home, by hiring UN employees essentially on the grounds of merit, by exerting fiscal orthodoxy, I hope to give more weight to my initiatives in the political field. . . . I believe the world needs the UN more than ever in these times of growing polarization and I will not preside over the decline of the organization, and I do not intend to be timid when it comes to preserving the peace and to protecting human rights.''