One night last month an Arlington police officer drove into the District, picked up an 18-year-old prostitute at a brothel near 14th Street and drove her back to an Arlington hotel. There, after the woman began to undress, she was arrested on charges of offering sex for money.

That drive across the Potomac illustrates how aggressively Arlington's vice unit has been pursuing prostitution, using techniques that have drawn increasing criticism from civil liberties groups. In the past 18 months, Arlington police have arrested 141 men and women on various prostitution charges. All but a few worked elsewhere -- either in the District, Alexandria, even suburban Baltimore -- and most came to Arlington at the specific invitation of police investigators.

"We don't care if these prostitutes live in other states or in D.C. If they want to engage in that kind of activity, let them stay there," says Arlington Police Chief William K. (Smokey) Stover. "We just want them to know they're not welcome in Arlington. We've got a fairly clean community here and we're going to keep it that way."

Stover's vice detectives usually have started their cases by scanning ads in the Yellow Pages or local sex-oriented publications for "discreet" escort, massage or modeling services. The officer makes a date to meet the woman -- or man -- at an Arlington hotel room or apartment that has either been leased by police or, in some cases, donated by the landlord or hotel owner.

With one officer hiding in a closet and the other playing the role of the "john," the scene would proceed -- first with an overt act by the alleged prostitute and then with the arrest. In most cases, the police would offer the prostitutes a deal: face jail or testify against the people who owned and operated the prostitution services.

This technique has snared an assortment of offenders -- from Harold Lyon, a bureaucrat with a $50,000 job at the U.S. Department of Education recently sentenced to nine months in jail on sex-related charges, to an illiterate woman and her 18-year-old daughter from suburban Baltimore sentenced to four months in jail for participating in a sexually explicit photographic session arranged by Arlington police.

These and other cases have outraged civil libertarians and defense lawyers, who charge that the Arlington police have come close to entrapment and even violation of the federal Mann Act, which prohibits taking women across state lines for immoral purposes.

"I think they should be concerned about crime in Virginia," said Arthur Spitzer, legal director of the Washington chapter of the American Civil Liberties Union. "I would think that they wouldn't need to bring it in themselves."

Drew Carroll, assistant commonwealth's attorney in Alexandria, said his office turned down a suggestion from vice officers in his city to adopt Arlington's tactics. "If we had a problem, we might consider it, but we don't want to make it our problem," he said. "It struck me as rather like inviting crime to come into your city." Montgomery County State's Attorney Andrew Sonner last year halted an Arlington-type prostitution operation in his county for similar reasons.

Arlington prosecutor Henry Hudson and Chief Stover, both veterans of the county's successful campaign seven years ago to ban massage parlors, vigorously defend their police practices, although Stover said he did not approve of his officers driving into the District and picking up the prostitute. "I have a problem with that tactic," said Stover, who spent 10 years on the vice squad. "I would have arranged to pay for her transportation to Arlington ."

Arlington officials say the recent surge in prostitution arrests in the county followed a rash of complaints from hotels in Crystal City and along Jefferson Davis Highway. But it is also due to the zeal of two detectives -- David Green and Tom Wise--who joined the the Arlington vice unit 18 months ago. In their first four months on the job they made 48 prostitution arrests, compared with six in the first eight months of 1980.

"Our policy was, if it involved sex for money, we'd make a case," said Green. "All we did was answer the ads." A 35-year-old officer, Green is praised by his Arlington colleagues as a tireless investigator whose motto was: "You're only limited by your imagination."

Wise and Green, now assigned to other units in the department, produced results. Charges were brought against owners and operators of almost a dozen Washington area outcall services--including Linda Daniels, Touch of Class, Dream Escort, Friendly Smiles and others. Some services closed -- at least temporarily -- as a result of the Arlington cases, the investigators say.

With that kind of record, few agencies now dare send women or men to Arlington. "The word I've gotten is that they're not going to take a chance going into Virginia," said Peter Randall, a vice investigator with the D.C. police.

"No," said a telephone receptionist for one escort agency advertising in the Yellow Pages. "We don't send our girls to Virginia. It's too far for them to drive."

Critics say that Arlington police have gone too far in their eagerness to arrest prostitutes. "Once they got the tiger by the tail, they were having too much fun to stop," said one defense attorney, who asked not to be named. "Generally, I thought they handled these cases with a little more enthusiasm than I am accustomed to," said Arlington lawyer Peter Baskin, who four years ago successfully argued an entrapment charge in a prostitution case in Arlington.

The case of the two Baltmore women was perhaps one of the most controversial. The two women, whose services were advertised in Met Personals, came to Arlington after police agreed to pay their $55 cab fare from Baltimore County. Leon Demsky, attorney for the mother, said his client was a poor, illiterate woman from the hills of West Virginia who had never heard of Arlington until the police lured her into the county last fall.

Demsky said the police were unnecessarily cruel to his client, taking more pictures -- close to 100, he said -- than were necessary to prove their case.

"They could have done the job without going to extremes," he said. "Why bother? Are the citizens of Arlington going to feel any safer because two misfits were brought here from Baltimore to commit a crime? It's one thing to stop a crime; it's another to create it."

Wise said the two women were arrested in cooperation with the Baltimore County police, who believed the Arlington police were in a better position to bring a case against the man who allegedly was offering the services of the mother and daughter. Charges were eventually filed against the women's employer and, according to Demsky, his sentence was suspended.

Across the river in the District, there are some who complain that the Arlington police are attempting to police Washington. "I can't believe the extent to which they go to put people in compromising positions and then arrest them for being in compromising positions," said Dennis Sobin, publisher of Met Personals, a monthly sex-oriented publication that has served as an Arlington police guide to available sexual services.

In one controversial case, Arlington police arrested two males dispatched by Friendly Models, an escort service based in Georgetown and then, armed with a search warrant issued by a District court, seized the establishment's business records--including a list of 500 to 1,000 preferred clients.

"I object to them imposing their values on our police," said Frank Kameny, a prominent gay activist in the District. "We have our own police department that is accountable to us, to our community standards."

Yet, some D.C. police applaud Arlington's efforts. "Right now, Arlington has an excellent prosecutor, the backing of the community and an excellent department," said vice investigator Randall. "Hats off to Arlington. We think it's great."

For their part, Arlington police have no apologies. "When someone advertises that they're violating the law, to me, that person is flouting the law," said Green. "All we did is provide an opportunity for people to violate the law that they said they were willing to violate."