For months, drug dealers sold an array of prescription drugs out of three Downtown McDonald's restaurants, operating in daylight and lingering for hours without buying food, an undercover police detective said.

The detective offered glimpses into a "lucrative" drug trade yesterday, a day after an employee of the Smithfield Street store was arrested for selling drugs in a rest room, and District Attorney Stephen A. Zappala Jr. threatened to take legal action against the restaurants.

Police Chief Nate Harper urged other Downtown business owners to take heed, saying they are "fully responsible to patrol their property."

More than 100 people have been arrested for selling or possessing drugs as part of a sting at the McDonald's restaurants that began in October, after patrons complained to police that they couldn't eat there without "being harassed," the detective said.

Police requested the undercover detective's name not be printed for his safety. He works in plainclothes out of the Hill District station.

Dealers, he said, offered a stock of prescription drugs and narcotics, including the painkillers Vicodin and morphine, as well as methadone and heroin. McDonald's employee John J. Eckard, 48, is charged with selling clonazepam, a muscle relaxant, and Xanax, an anti-anxiety drug, at the Smithfield Street restaurant.

The detective said some dealers would acquire a prescription of 90 pills for just $1, selling each pill for as much as $7, in transactions that generated hundreds of dollars daily. Police are investigating where and how they got the drugs.

Sales often would run from 8 a.m. to noon, until dealers shifted their hours in an attempt to elude officers and police informants who were doing undercover buys.

"It was a chess match between us and them," the detective said. Detectives made two or three arrests per day, some people more than once.

"They used the McDonald's because it was easy for them to sit there for hours at a time without anyone telling them to leave," the detective said.

Employees at the Wood Street location declined to comment. McDonald's, which also operates on Stanwix Street, issued a statement.

"Operating safe restaurants will always be a priority for McDonald's. ... We require employees to be in full compliance with local, state and federal laws, as well as adhere to our own stringent employment practices."

Waiting for his bus outside the Smithfield Street restaurant last night, Richard Webb, who eats there daily, said the drug sales are a well-known "fact." Some dealers, he said, swap cash for drugs while waiting in line. Others sell openly at the tables, and still others sell outside.

The undercover investigations will continue, the detective said, declining to specify where.

"The word around methadone clinics is that Downtown is hot, and you've got to find another place," he said. "But they haven't yet."