Capping an 18-year battle over beautifying Montgomery County roadways, the County Council voted yesterday to ban billboards and tear down the last 56 such signs left in Montgomery.

County officials -- following a national trend to curb billboards that began with Lady Bird Johnson's efforts to clean up American highways in the 1960s -- have been trying to eliminate the big signs since 1968 but have met stiff opposition from the company that owns most of them. The new law, a zoning text amendment that does not require approval of the county executive, requires that the existing billboards, which are distributed throughout the county, be dismantled immediately.

The measure, which passed 5 to 0 by the council with two council members abstaining, targets so-called "off-site advertising" and does not affect smaller political or noncommercial signs.

"Some of the billboards are just in the way," said council member Rose Crenca, one of the bill's supporters. "In some of the county's major intersections, they just add to the cluttered look and the confusion when you're trying to find your way."

The other members voting for the measure were David L. Scull, Scott Fosler, Neal Potter and Michael L. Gudis.

Esther P. Gelman said she supported the billboard ban, but abstained because she had questions about how the measure affected First Amendment constitutional rights. She also was prohibited by law from voting because she had not attended the public hearing on the issue or read the transcript of that hearing.

Kathleen Hart, an assistant county attorney, said that the measure does not violate First Amendment rights because smaller noncommercial signs will still be allowed.

Council President William E. Hanna Jr. also abstained from voting, saying that, although he wanted strong controls over the placement and design of billboards, he did not support their abolition.

"I don't ever remember us legislating one entire class of industry out of existence in one fell swoop," Hanna said. "That's a pretty drastic step."

Rollins Outdoor Advertising Inc., which owns 48 billboards in the county, has fought to keep its signs since 1968, when the county passed a law that allowed billboards only under certain conditions in specific industrial zones. None of the Rollins billboards met the conditions, and the company was ordered by county officials to remove them by 1972. But the order became tangled in a legal web when Rollins appealed the case to the county Board of Appeals and the Montgomery County Circuit Court.

Norman Glasgow, the attorney for Rollins, said he does not know whether the company will appeal the council's action in court.

Tom Somers, general manager of Rollins, criticized the measure. "Billboards are the most economic form of advertising for the small businessman," he said. "The rental charge for a billboard is about $500 a month, and it costs that much to place one ad in the newspaper."

During the past 20 years, the battle over billboards has pitted environmentalists, who argue that the signs contribute to visual pollution, against those who contend a billboard ban violates First Amendment rights.

In a 1981 San Diego case, the Supreme Court indicated that laws banning all billboards might be permissible under the Constitution. But the justices' opinions were so varied that they offered little guidance to jurisdictions, court observers said. Several cities and states have almost totally banned billboards. Others have more limited regulations.

"Typically across the country, billboards are severely regulated and confined to commercial and industrial areas," said Eric Rubin, a Washington lawyer who represents billboard companies.

Rollins has also challenged billboard regulations in the District. For about five decades, most Washington officials believed city regulations prohibited billboards. But a Rollins challenge in 1978 resulted in regulations in 1980 permitting the signs in industrially zoned areas throughout much of Washington, particularly in Northeast.