District of Columbia police have started sending undercover narcotics officers into the city's high drug trafficking areas to sell narcotics to willing buyers and then arrest them, apparently the first such effort in the nation.

Operating with the approval of U.S. Attorney Charles F. C. Ruff, the police began the experimental operation late last week and have already made a dozen arrests, according to one police official.

Police regularly purchase drugs from dealers who can then be arrested and charged, but this police operation is unusual because the police apparently have never before acted in the role of "pusher" by first selling the drugs and then making arrests.

"We have to try other than traditional methods," said Inspector Kris Coligan, director of the police departmant's morals division. He said police were reacting to the strong citizen outcry of open narcotics purchases on city streets.

Coligan first proposed the plan several months ago to Ruff, who gave his approval, but instructed that police were to avoid any actions that might constitute entrapment.

"This [a drug purchase] is not something that will be initiated by the police," Ruff said in an interview. "We are cognizant of the implications of entrapment and other due process issues. Entrapment really requires inducement by a police officer and an absence of predisposition by the defendant. We are very careful of not . . . forcing an unwilling buyer to purchase narcotics."

Yesterday, however, after what apparently was the first arraignment in D.C. Superior Court of one of the individuals arrested under the new scheme, defense attorney Robert J. Pleshaw claimed that his client, William McLaughlin of Wheaton, had been the victim of "entrapment, like the Abscam cases."

Police have alleged that McLaughlin "purchased one yellow pill [the pain killer Dilaudid] from an undercover officer of the metropolitan police department at Columbia and O streets NW" on Jan. 22. Dilaudid is a synthetic form of heroin.

Police documents allege that McLaughlin paid $30 after he "asked an undercover officer if she had a 'D'." McLaughlin, who pleaded not guilty to the misdemeanor charge, said after the arraignment that "stopping me is not going to stop the man they are trying to get."

Police declined to disclose the exact number or location of the undercover officers, but stated that it might number in the "dozens," if necessary. A police official said that traditionally high drug-use areas, including such locations as Ninth and 10th and O streets NW, 12th and U streets NW, 14th and P streets NW, Wheeler Road SE, and Martin Luther King Jr. Avenue and Talbert Street SE.

According to Coligan, the plan calls for undercover narcotics officers to be supplied with previously seized narcotics and then sent to blend in with known dealers in the high drug-trafficking areas.

"When approached by would-be buyers who would then stripe up a conversation as to what . . . they are looking for, such as Dilaudid, Preludin, and cocaine," Coligan said, "we would more or less act as an avenue to carry out their predisposed plans of obtaining narcotics."

Some lawyers yesterday expressed concern that the plan might violate defendants' rights. "If the cops are walking up and down the street and saying to people who they have no reason to believe have bought drugs before, 'Do you want to buy drugs?,' we would be bothered by that," said Arthur B. Spitzer, legal director of the American Civil Liberties Union here.

"One of the things that bothered us about the Abscam situation is that police seemed to have targeted people who there was no reason to believe were already engaged in criminal conduct, and set them up."

Spitzer said, however, that if police merely posed as pushers, walked in neighborhoods where they knew a lot of drugs were used and waited for people to approach them, the ACLU likely would not object. Ruff said that is the procedure being used by police.