Arkady Nikolaevich Shevchenko made his final break with Moscow last night.

The highest-ranking Soviet official ever to defect -- now a high-priced speaker, essayist and author of the best seller "Breaking With Moscow" -- took the oath of U.S. citizenship in front of a distinguished group of friends at the Mayflower Hotel.

Afterward, at a party at the hotel attended by Sens. Daniel Patrick Moynihan (D-N.Y.) and Malcolm Wallop (R-Wyo.), Rep. Les Aspin (D-Wis.) and FBI Director William Webster, Shevchenko heard himself welcomed to his new nationality in a "Dear Arkady" letter from Ronald Reagan.

Shevchenko, 56, the erstwhile Soviet undersecretary to the United Nations, declared the occasion "one of the finest moments of my life."

"I must say," he read from a prepared statement, in a pronounced Russian accent, "that I have a pang of sadness in remembering my native land. I was more than glad to break with the Soviet regime, but not with the Soviet people, who suffer under communist oppression."

The shindig was tossed in a big ballroom by the Jamestown Foundation, a group devoted to easing the rites of passage for important Soviet defectors. There were press kits and open bars, a platoon of hors d'oeuvres-toting waiters and PR functionaries ("Please mention," said one, "that we're a Stouffers hotel") -- and, up front, an American flag that looked as though it had been commandeered from the set of "Patton."

Someone set up a display table of Shevchenko's book in various foreign-language editions: Italian ("Addio a Mosca"), Spanish ("Ruptura con Moscu'"), French ("Rupture avec Moscou"), German ("Mein Bruch mit Moskau"), Finnish ("Va lirikko Moskovan"), Swedish ("Att Bryta med Moskva"), Danish ("Brud med Moskva") and Russian (not reproducible in the Roman alphabet, alas).

"The French always seem to have the best titles," mused Shevchenko's editor from Knopf, Ashbel Green. "I was once at Peter Benchley's house, and I noticed that the French edition of 'Jaws' is called 'Les Dents de la Mer.' "

The citizenship ceremony took place in somewhat more sedate surroundings.

"Are you ready?" U.S. Circuit Judge Alex Kozinski, himself an e'migre' from Romania, asked Shevchenko in a hushed meeting room at the hotel.

"I am," Shevchenko answered in a firm voice -- all the better to be recorded by the tangle of microphones sprouting from the wall of TV cameras.

The two stood, along with Shevchenko's mother-in-law, June Huff, and a bow-tied government lawyer sent by the Immigration and Naturalization Service. The judge wore his robes. The man of the moment, sporting somber grays, laid a pale hand on a Bible held by his American wife, the former Elaine Jackson, whose festive peach suit perfectly matched the wallpaper.

Then -- in a brisk cadence set by the judge -- he promised to "renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state or sovereignty," to "defend . . . the United States of America against all enemies" and to "bear arms . . . when required by law," among other patriotic duties.

At the end there was lusty applause from the likes of Wallop, Rep. Tom Lantos (D-Calif.) and Arnaud de Borchgrave, editor of The Washington Times. Shevchenko applauded back. One man, moved, produced a handkerchief to wipe his eyes.

"I feel wonderful," Elaine Shevchenko said.

"I hate this word 'defector,' " Shevchenko told his supporters. "Because it has some meaning that personalities are defective in certain ways."

Citizen Shevchenko, as people were calling him last night, defected in April 1978, after spending several years -- he revealed in his book -- as an American spy. A month after his defection, his first wife, Lina, who had been spirited back to Moscow along with their teen-aged daughter, died of a drug overdose (a suicide, the Russians said). A few months after that, a call girl named Judy Chavez announced that Shevchenko had paid her for companionship with money from the CIA. He admitted the relationship but denied that the money, about $40,000, had come from the Agency.

After marrying Elaine in December 1978, Shevchenko's life calmed down considerably. He has managed -- as his lecture agent, Joe Cosby, has said -- to go "from communism to tax shelters in seven years." The CIA reportedly pays him a $60,000 annual stipend.

Now that he is a citizen, of course, Shevchenko can vote, but the question remains as to whether he'll register as a Democrat or a Republican.

Said Republican Wallop, "He's probably still Russian enough to be an independent while not wanting to tip his hand."

"It's his choice," said Democrat Moynihan. "Why don't you ask him?"

Shevchenko, cornered, was noncommittal.

"I don't know who will be the candidate," he said. "The party depends. For me, more important the personality.