The Fairfax County Board of Supervisors yesterday proposed a law to require nonsmoking sections in most county restaurants, prompting restaurant and tobacco industry officials to vow they will fight it in court.

"We oppose any type of government regulations and are going to take every means possible to fight it," declared Preston H. Nebel, president of the local chapter of the Virginia Restaurant Association, moments after the board approved a public hearing on the proposal.

Nebel said that although most restaurant owners could live with the proposed law, they fear it would clear the way for more far-reaching demands, including the addition of rooms to further separate smokers and nonsmokers.

The proposed ordinance, sponsored by Supervisor Thomas M. Davis III, would require restaurants with a seating capacity of more than 75 people to cordon off sections for nonsmokers. Failure to comply would result in a $25 fine.

Davis dismissed arguments that the law would be costly to business people, saying the county was not asking them to build separate rooms but merely to segregate smokers and nonsmokers with planters and other devices.

Asserting that public support for the measure was "overwhelming," Davis disputed arguments that the thrust of the proposal was antibusiness and said, "All we're trying to do, for the first time, is to recognize the rights of nonsmokers."

Davis introduced the proposed ordinance after receiving assurances from the county legal staff that the measure could withstand a court challenge. A smoking ban already is on the books in Fairfax County for all food stores and elevators.

(Montgomery County has passed a bill to restrict smoking in public and commercial places, but it does not apply to restaurant owners who have said they would voluntarily provide nonsmoking sections. The District of Columbia has a ban on smoking in public gathering places but it does not cover restaurants.)

Davis' proposal ran into opposition from Supervisor Nancy K. Falck, the lone smoker on the nine-member board of supervisors, and board Chairman John F. Herrity, who said he quit smoking after his most recent heart attack.