Fairfax County plainclothes police officers infiltrated a high school graduation party at a Vienna home early yesterday, without a warrant, and subsequently arrested several participants, including the hostess, who was handcuffed, charged with being drunk in public and forced to spend the night in jail.

Police said the plainclothes officers, and agents of the Virginia Alcoholic Beverage Control board, showed up at the party at the house of Robert and Retha Morgan because uniformed officers on patrol Thursday night said they spotted at least 50 teen-agers milling around the neighborhood and said that some teens appeared to be drinking and others appeared to be drunk.

"I'm confused," said Retha Morgan, who was charged, as was her husband, with the additional count of aiding and abetting the consumption of alcohol by minors, according to Fairfax County police.

"I'm very confused. I don't know where the thing went wrong," she said. "I have played it over and over again in my mind. I really wish I could understand the police's side and what they thought they were doing."

Police said yesterday that it has long been the practice in Fairfax County to send plainclothes officers to parties where officials have reasonable cause to believe that the law is being broken. They said they were told by officials in the local commonwealth attorney's office that such action is permissible if a party appears to be public, with no one at the door to check for invitations or to turn away strangers.

Commonwealth's Attorney Robert F. Horan Jr. could not be reached for comment yesterday.

Virginia ABC officials also were unavailable.

Fairfax police said the party at the Morgans' house, in the 10000 block of Old Hunt Road, was one of several visited by plainclothes officers and ABC agents Thursday night and yesterday morning as part of what Capt. Richard J. Rappoport, commander of the Reston Police District, said was a joint effort to crack down on illegal drinking by teen-agers attending graduation celebrations.

Rappoport said that "the way we handled the {Morgans'} party is not atypical from the way we handle parties all the time."

But he stressed that parents giving "well-controlled" parties where there are "no obvious violations of the law, such as people drinking in the street or juveniles carrying alcohol in and out of a party," should not fear that police will surreptitiously go onto their property.

Several law school faculty members knowledgeable about constitutional law said yesterday that, in their view, the legality of the action taken by the Fairfax police hinged on the partygoers' expectations of privacy and the degree to which officers had reason to believe that serious illegal acts were taking place on the Morgans' property.

"That's a tough one," said John F. Banzhaf III, a law professor at George Washington University.

Morgan said she thought that she and her husband had planned a perfect graduation party for their son, Kurt St. Clair, in the back yard of their house. They hired a disc jockey, bought hot dogs and soft drinks and invited Oakton High School's entire senior class of 619 students.

Morgan said she informed police of the party and even received a booklet of party tips from them. She said that she posted no-alcohol signs and that she and others did their best to control entrance to the party and to ensure that the 150 to 200 teen-agers who showed up were not carrying alcohol.

Morgan said the party was not rowdy and did not bother neighbors -- a claim that several neighbors interviewed yesterday agreed with. Sometime during the merrymaking, the plainclothes officers joined in. Rappoport said there were two or three from his department, plus a few from the ABC board, at least one of whom was not in uniform.

Rappoport said his officers were never asked who they were and did not offer the information. He added that it quickly became apparent to them that "a number of kids were drinking, a number were intoxicated, there was no control of the party, and the adults who appeared to be in some position of supervising the party were not exercising any control over it. People were drinking in and out of the party and on the street."

As the teen-agers became aware of the police presence, several were scared, and others became angry, using profanity, Robert Morgan said. A few juveniles were then charged with under-age drinking, police said. It is not clear how many were charged because some were charged by the ABC agents.

Rather than end the party then, Retha Morgan said, she thought it best to extend the music for another hour -- until 2 a.m. -- to give the teen-agers a chance to settle down, drink coffee and find rides home, if necessary.

As the party broke up, several of the teen-agers were stopped at a nearby police roadblock; two were charged with traffic infractions and a few more were charged with under-age drinking, police said. It was at 3:15 a.m., when Retha Morgan walked up the street to the roadblock to find out what was going on, that police asked her to take a sobriety test.

Morgan told a reporter yesterday that she had had one scotch before her son's graduation and had just finished a second before she approached the roadblock, empty glass in hand. She said that while she was legally drunk, according to her performance on the sobriety test, she was in control of her actions.

Police charged her with being drunk in public, and, as part of standard procedure, they handcuffed her and took her to a magistrate, who ordered her to spend the remainder of the night in jail.

Police officials contacted in other jurisdictions yesterday expressed surprise at the tactics employed by the Fairfax officers.

"Not to criticize them, but it's not the normal procedure or approach we'd take," said a District police official who asked not to be quoted by name.

Montgomery County police spokesman Harry Geehreng said his department does not use plainclothes officers to infiltrate teen-agers' parties.

Another Montgomery police official, who asked not to be identified, said, "Personally, I'm a little shocked. We've got better things to do."

Staff writers Joseph E. Bouchard, Victoria Churchville, Charles W. Hall and Lynda Richardson contributed to this report.