"Even a party like this is a working session."

George K. Dunye, finance minister of Liberia, stood near the East Garden Court at the National Gallery of Art, looking down the marble sculpture hall jammed with nearly 2,000 delegates from the 36th annual meeting of the World Bank and the International Monetary Fund.

Dunye, who heads his country's delegation to the week-long meeting in Washington that has drawn delegates from more than 140 countries, was quite serious about a party turning into a working session.

"It's a place to meet people and know where to go when you need help on a project," explained Dunye, later to be seen chatting to a few gray-suited American banking officials.

Dunye and other delegates from the developing nations heard President Ronald Reagan open the meeting Tuesday with an address urging them to believe in the "magic of the marketplace" and to rely more on private enterprise than on foreign aid to build their economies. It was a thesis that met with what was described as only polite applause.

"It was a matter of not being offensive, to us," Dunye observed.

"It was what we expected from the administration and its policies," he added. "We knew it would not be the old way of direct cash. It will be aid for identifiable projects that the administration feels viable. To me, that's economically sound."

In the party atmosphere, the delegates offered little criticism of President Reagan's speech, which was seen by some observers as a signal that the United States would not increase aid to the Third World in these uncertain economic times of high inflation and budget deficits.

If any belt-tightening was taking effect last night, it was in the receiving line. While their guests munched on hot pastries, meatballs, raw vegetables and fresh fruits, hosts Paul A. Volcker, chairman of the Federal Reserve, and Treasury Secretary Donald T. Regan and their wives were still standing in the receiving line nearly two hours after the reception began.

"I need the belt-tightening," said the treasury secretary, pulling in his stomach muscles.

When someone remarked that the National Gallery was a grand place to give a party, Barbara Volcker looked down the elegant marble corridor lined with sculpture and asked:

"How do you like my collection?"

Defense Secretary Caspar Weinberger came to his fellow Cabinet officer's reception a little late. Had he been on the Hill or arguing the administration's policy on the MX missile?

"No, I was in Annapolis today for a parade," he explained. "I love a parade."

Guests enjoyed the music of a three-piece combo and wandered into adjoining galleries to see the Manets, Monets and Picassos.

Everyone is conscious of the call for belt-tightening these days, it seems. In response to a question about the cost of the party, a Treasury Department spokesman pointed out that the National Gallery was free for the use of the treasury secretary and the food really didn't cost that much.