The Federal Communications Commission yesterday said it would enforce a 24-hour ban on the broadcasting of "indecent" programs by the nation's radio and television stations in a move it said was designed to protect children from questionable programming.

The action comes amid a series of recent battles involving government efforts to regulate or ban controversial material, including the ongoing fight over funding by the National Endowment for the Arts (NEA) and the banning in Florida of a record by the rap group 2 Live Crew.

Although the ruling does not affect programs that run exclusively on cable television, it covers programming of all local radio and TV stations and the three major networks. Networks collectively attract nearly 70 percent of TV viewership on any given night.

If it withstands a final ruling by an appeals court and an anticipated challenge by broadcasters, the ban would enable the agency to levy fines against broadcasters who air material considered indecent under a definition adopted by the Supreme Court in 1978.

The FCC's decision immediately raised questions about what material would be subject to government action. Asked by reporters if such relatively daring network programs as "L.A. Law" and "thirtysomething" could face government scrutiny, FCC general counsel Robert L. Pettit said, "It's impossible to say in the abstract. We'd have to see it first and judge it on a case-by-case, complaint-by-complaint basis."

The ruling defines indecency as "language or material that, in context, depicts or describes, in terms patently offensive as measured by contemporary community standards for the broadcast medium, sexual or excretory activities or organs." Material deemed "obscene" by a similar test is banned from the air at all times.

Before yesterday's ruling, the ban on indecent material only applied to programming aired between 6 a.m. and 8 p.m.

In an indication of what the FCC might consider indecent, it previously cited a Kansas City, Mo., television station that aired "Private Lessons," a movie about a housekeeper's seduction of a 15-year-old boy, in prime time. It also cited four radio stations that broadcast songs and skits about homosexual sex acts, penises and masturbation. Since 1987, the courts have upheld only one FCC fine for indecency, against a Los Angeles public radio station that aired a play about homosexuality during daytime hours.

The enforcement actions of recent years have had "a chilling effect" on broadcasters, said Timothy Dyk, an attorney who represents a coalition involving the three networks, other broadcasters and public-interest groups that has opposed the 24-hour ban. "Broadcasters have been saying to themselves, 'When in doubt, leave it out,' " said Dyk. "There are large areas of doubt {in the ban}. The effect has been self-censorship. No one wants to be fined by the FCC." As such, he predicted yesterday's decision would not significantly change radio and TV programming practices.

But the FCC maintains that some indecent programming continues to be aired and the ruling is needed to bring enforcement action against offenders.

A spokesman for NBC said the network had not seen the ruling and would not comment. Representatives of ABC and CBS could not be reached.

FCC Chairman Albert Sikes tried to draw a distinction between the FCC's action and the NEA and 2 Live Crew controversies. "Those are forms of censorship that are far more corrosive than what this commission is doing," Sikes said. "The purpose of this is to protect children from indecency on the airwaves. It would be wrong to define this as some sort of knee-jerk reaction to the religious right."

Congress ordered the 24-hour ban two years ago, but a federal court, responding to a challenge by Dyk's group, put FCC enforcement of it on hold while the FCC compiled evidence to support its constitutionality.

The FCC said yesterday that an all-day ban does not violate First Amendment guarantees because it promotes a "compelling" government interest and is "narrowly tailored" to serve that interest. The Supreme Court has ruled, in a case involving a telephone "dial-a-porn" service, that indecent speech may be banned if a law meets these two tests.