The Ronald Reagans of 1600 Pennsylvania Ave. gave their first formal party last night with an international cast and Nancy Reagan back in her inaugural gown.

For all the men, diplomats from 141 countries, it was white tie and decorations and for the women, white gloves and jewels. The party climaxed a day of high spirits and patriotism that saw President Reagan preside at a "welcome home" ceremony for the 53 freed American hostages on the South Lawn of the White House.

Secretary of State Alexander Haig Jr. was euphoric about the day's celebrations and the lessons it offered to countries bent on terrorist tactics.

"Any terrorist nation would have to be impressed by the changing mood of our country and by the words of President Reagan when he said, 'Our patience is not inexhaustible,'" Haig told reporters gathered around him in the East Room.

As for Reagan's phrase, "swift retribution," in his speech to the freed hostages, Haig said, "It means what it says," then laughing mischievously, added, "Studied ambiguity."

President and Mrs. Reagan received the diplomats in the Blue Room where they filed two by two on through to the Green Room and waiting handshakes from Vice President and Mrs. George Bush and Haig and his wife, Pat. Some of the guests wore national dress, but probably none stood out more than the ambassador of Tonga, Inoke Fotu Faletau. His outfit consisted of a short, two-piece light blue "dress" with a beige cummerbund, called a taovala. His shoes were sandals and his legs bare.

"You've got nice legs, anyway," someone noted.

"Thank you," he said, blushing slightly. "You catch me at an unfair advantage."

No one was wearing a yellow ribbon.

After everyone had gone through the line and was in the East Room sipping California champagne, the Reagans turned up to get a little better acquainted with their guests, most of whom they had never met before. But no one knew how to begin, and for a few seconds, there was an uncomfortable silence. Then Mrs. Reagan started talking about how it had been "quite a day," and the president just listened, smiling.

Ambassador and Mrs. Roland de Kergorlay of the European Community were standing nearby when Reagan struck up a conversation with them.

"I was so impressed that everyone was speaking our language, and I couldn't speak any of theirs," the president said. "I knew a few words of high school French, but I decided not to try." It brought back memories of an earlier venture into foreign tongues. He told of traveling with friends in France some years ago and arriving hungry in a small village. They were looking for a place and spotted a gendarme.

"I said, 'J'ai faim,'" he said in a passable accent. "And I asked, 'Ou est le restaurant?' But when he answered, I didn't understand a word he said."

The dean of the corps, Soviet Ambassador Anatoliy Dobrynin, led off the diplomats, who included the Cuban Interests Section's Ramon Sanchez-Parodi, the first representative of his country to attend a White House function in 19 years.

"I welcome it," said Dobrynin of Sanchez-Parodi's presence, but declined to discuss other political topics.

Algerian Ambassador Redha Malek was equally mute about Secretary of State Haig's decision to sell arms to Morocco, which is at war, with the Algerian-supported Polisario guerrillas in the Western Sahara.

"Excuse me, my job is very difficult," said Malek of the touchy question of arms supplies. Malek, who played a significant role in negotiations leading to the release of the hostages, was prominent in yesterday's welcome home ceremonies at the White House. "I feel very honored," he said last night.

Moroccan Ambassador Ali Bengellun was buoyant, but careful about his words."It took a long time to get this kind of action," he said, declining to speculate on why the deal was held up until now. "I'll let you do the guessing," he said.

Haig said it was "not at all" an affront to Algiers, but instead "an affirmation of our historic ties" to Morocco, despite a report that former deputy secretary of state Warren Christopher had counseled Haig to block the arms sale as a favor to Algeria for its help in the hostage crisis.

On the Carter administration's agreement with Iran, Haig said, "We're stuck with it."

But not all the talk was so heavy. The Swedish ambassador, Wilhelm Wachtmeister, said, "We have a close connection with the White House -- in the sense that Mrs. Reagan's hairdresser is a Swede." Which led someone else to note that Mrs. Reagan was not wearing her swept-back inaugural hairdo with her inaugural gown by Galanos, which is destined to go into the Smithsonian.

The White House said last night's reception was the first white-tie event for diplomats since the Gerald Fords lived in the mansion. It pointed up the contrast in style between the Reagans and their predecessors, the Carters, at least in matters of dress.

As for a comprehensive view of the Reagan style, it was still too early to tell. "It's very tough to come into a house and bang, throw a party," said Letitia Baldrige, social secretary to the John F. Kennedys and temporarily undertaking that task for the Reagans. "But this is a very easy party to give because there's no hot food -- it's a dessert-type thing."

And on that score, Baldrige said Nancy Reagan had had her say. "She cares about nice things to eat."