Even by Washington's Very Social Standards, it was easily one of the largest collections of names and egos assembled under one roof since the last inauguration.

Saudi Arabian Ambassador Faisal Alhegelan and his wife, Nouha, threw themselves a farewell cocktail party last night at the vast Organization of American States building, and more than 1,500 guests came.

It was a bit crowded.

"Everyone is here--even my neighbors," said Defense Secretary Caspar Weinberger.

The Alhegelans became one of Washington's most desirable couples nearly the moment they arrived in 1979, and soon after surprised insiders when Nouha Alhegelan became an outspoken advocate for peace in Lebanon.

As a rather unconventional ambassador's wife, she led the Arab Women's Council campaign to "humanize" the war by staging vigils and hunger strikes in front of the White House, and turned her back on social Washington. She chose instead to spend her money on the Arab world crisis.

"She did an enormous amount of work in this country," said Lorraine Cooper, who came with her husband, John Sherman Cooper, a former senator and ambassador. "Everyone loved her before and understood why after."

"She's a very courageous woman," said Jayne Ikard.

The two-hour-long receiving line in the grand hallway, which trailed down marble stairs and emptied outside, included Attorney General William French Smith, Energy Secretary Donald P. Hodel, former CIA chief Richard Helms, former Reagan national security adviser Richard Allen, Sen. Edward Zorinsky (D-Neb.), Sen. John Tower (R-Tex.), Sen. Charles Percy (R-Ill.), journalists Nancy Dickerson, Marvin Kalb and Chris Wallace, and the ambasadors of Italy, Spain, Germany, Denmark and Switzerland.

Hot rumor of the night: A PLO representative was lurking about. "His wife is wearing a purple dress," offered a photographer. "I heard he was over there by the shrimp," whispered a woman in a red dress.

The suspect was never found.

The elaborate buffet table, catered by Georgetown's fashionable Lansdowne, was the size of about five Ping-Pong tables and seven-deep with people. You couldn't get anywhere near the veal.

"A pound of food per person," explained Gerry Croce of Lansdowne. He looked frantic. But organized.

Percy, chairman of the Senate Foreign Relations Committee and facing what is expected to be a rough reelection bid next year, was almost as swamped as the Alhegelans.

"No, I'm not worried," he said. "I'm not losing any sleep over it. It's not a tough race."

Toward the evening's end, Nouha Alhegelan, who has been ill, sat down to welcome remaining guests in the receiving line. But she kissed almost everyone and never forgot the first names.

"My biggest accomplishments here," she said, "are my friendships."

And what will she miss the most?

"My children, of course. They will all stay"--in school.

She then walked down the grand staircase. And on to Paris.