Sixteen persons, including a movie screenwriter, a British nobleman, the ex-husband of British singer Marianne Faithfull and a former New York City police detective, were arrested Thursday and charged with conspiring to smuggle heroin and cocaine into the United States.

The complaints, filed in U.S. District Court in Brooklyn, allege that the multimillion-dollar narcotics ring was run out of a lavish town house owned by Mik Cohn, 37, a British free-lance writer who specializes in rock music and who wrote the screenplay for "Saturday Night Fever." The four-story town house is located at 24 West 76 St., in a fashionable residential section of Manhattan less than a block from Central Park.

Federal sources say that the town house has been the scene of wild parties frequented by "jet-setter types" and that traffic into and out of the neighborhood continued "at all hours of the day and night."

Besides Cohn, those arrested include John Jermyn, 29, who is a British earl, and Ian Ben Brierly, 32, a punk rock musician who uses the stage name Ben E. Ficial and was once married to Faithful. Another defendant is Frederick Albano, 42, a former New York City police detective.

Jermyn, who is listed in Who's Who as Earl Frederick William John Augustus Hervey Jermyn, son and heir of the Sixth Marquess of Bristol, was identified by the Drug Enforcement Administration from his picture in a recent article in New York magazine entitled "The Earl Jermyn," which described him as having extensive oil and farm holdings in the United States. Jermyn, who lives in a $1.5 million Sutton Place apartment here, is reported to have a fortune of $7.5 million.

According to the complaints, the ring was run from Cohn's home by Frinces Mullin, 32, a British woman who was convicted in 1979 of smuggling hashish from India to Sweden. Mullin lives in a two-story apartment in the Cohn town house.

The DEA started its investigation last December after the arrest of Laurita Watson, a drug courier who is now cooperating with federal authorities. Watson, who was arrested December 20 at Kennedy International Airport while carrying four pounds of heroin, has told authorities she was recruited by Mullin to bring heroin into the United States from India and cocaine from Brazil.

Andy Pucher of the DEA said that the case was based largely on court-authorized wiretaps of 3,500 telephone conversations on Mullin's telephone lines since last Feb. 18. He said 40 percent of those conversations were related to drug trafficking.

Despite the December arrest, members of the group continued to discuss drug dealings on the telephone using code words like "chocolate" and "script" for heroin and "Charlie" and "Chinese food" for cocaine, according to the complaints.

Several conversations with alleged heroin sources in New Delhi, India, were recorded. The wiretaps detail numerous conversations between Cohn and Mullin, including one in which Mullin told Cohn, "I have a package" and Cohn replied, "That would stop me from dying."

Cohn told reporters he was innocent, and his attorney, Andrew Maloney, told the court that Cohn is "a user and abuser of drugs, but he is not a trafficker."

The wiretaps also show extensive communications between Mullin and Jermyn. The complaints state that Jermyn telephoned Mullin and said, "You've got some of my cash or credit." Mullin responded, "Yes, I do. I'm putting it towards, so I can get you the cheaper price."

Alan Vogeler, a spokesman for Jermyn, today answered a phone call to a number that the DEA picked up in its wiretaps of Mullin's conversations.

Vogeler said: "The charges are outrageous and without foundation, and the Earl Jermyn categorically denies that he was involved in any conspiracy to distribute narcotics. It appears he has been mistakenly dragged into a case against an alleged drug distributor. The earl is certain that he himself will be vindicated by subsequent events and looks forward to the opportunity to prove his innocence."