"I hope Mrs. Reagan will forgive me for stealing her line," said Helene von Damm, longtime aide to President Reagan, who was standing a few steps away on the platform, "but my life, too, began with Ronald Reagan."

Nancy Reagan wasn't there yesterday ("She was busy in private appointments," said Sheila Tate, her press secretary) but just about everybody else in the Reagan administration and their spouses were as von Damm was sworn in as U.S. ambassador to Austria. Administering the oath was her friend and colleague, national security adviser William Clark.

The extraordinary guest list of 300 included the vice president, half the Cabinet, senior White House staff members, agency heads, State Department officials, members of Congress, dozens of lesser-known Reagan appointees, von Damm's former staff from the Office of Presidential Personnel, and even her hairdresser and seamstress. And in another break with precedent, instead of the customary State Department setting, ceremonies were held in the State Dining Room at the White House.

"In looking back, what I cherish most about the American people is their incredible generosity. With all the handicaps of a poor immigrant, I never felt anything but total acceptance and support as I tried to make my way," said the Austrian-born von Damm, a naturalized America citizen who began working for Reagan during his campaign for California governor.

That was true most of all, she said, of Reagan, "who always found it perfectly normal to hear an Austrian voice on the intercom."

Reagan called her "the most American human being I ever met. She made all of us admire the people of Austria. She was more American than Patrick Henry, and the few times I saw her bristle was when someone was criticizing this country. She would even stomp her foot."

Reagan said he had greeted the proposal that Von Damm be named ambassador to Austria "with mixed emotions--I greet this day with mixed emotions. How do you say goodbye to someone who has been so important in your life for 16 years?"

Reagan expressed confidence in von Damm's ability to represent the U.S. and said that one particular thing added to his "joy--that she will have a chauffeur to drive her around.

"The only American thing she didn't master was traffic. She has a couple of totaled Porsches in testimony," said Reagan to uproarious laughter. "We still have a string on you, Madame Ambassador. Bless you."

With von Damm was her husband, New Jersey businessman Byron Leeds, who will accompany her to Austria early next month.