The Alexandria City Council, joining a trend to restrict smoking in public places, last night gave tentative approval to an ordinance that would require restaurants to set aside at least a fourth of their seats for nonsmokers.

On a unanimous vote, council members endorsed a proposal that would require all existing restaurants that seat more than 40 people to establish no-smoking sections. Existing restaurants would have to designate at least 25 percent of their places for nonsmokers; restaurants built after the ordinance takes effect would be have to earmark half their seats as smokeless.

The council must take a second vote for the ordinance to take effect, but the measure attracted no opposition last night and council members predicted that it will pass easily. Final action is scheduled for late this month.

The council's action reflects increasing concern about the health effects of tobacco fumes not only on smokers but also on people who are in close quarters with them. Fears about so-called passive smoking have led several Washington area localities to require no-smoking sections in restaurants.

Federal aviation authorities have banned smoking on some commercial flights.

Vice Mayor Patricia S. Ticer, a Democrat, said after the council voted that she was sorry that the city had not approved the no-smoking requirement sooner. "It's a matter of consideration for others," Ticer said. "This is appropriate given the profile of the American public today.

"We were hopeful that the restaurant owners would be more sensitive to the public voluntarily, but they were not. This should help."

Council member Carlyle C. (Connie) Ring Jr., a Republican, said, "Equity dictates that you afford a relatively smoke-free atmosphere for those who reasonably may wish it. Restaurant owners can now say, 'I'm providing {a nonsmoking section} because my customers want it and the law requires it.' "

Washington area counties that require no-smoking sections include Fairfax, Arlington, Prince William, Montgomery and Prince George's.

The move to require no-smoking sections in Alexandria began almost two years ago when city officials appointed a committee to study the issue.

In a report issued last June, the task force said that previous city requests for restaurants to establish the smokeless sections voluntarily had yielded unsatisfactory results. Only 25 percent had done so.

The committee recommended that the cafe owners be required to act.

"Nonsmoking patrons of the city's many fine restaurants ought to be able to enjoy their meals as free as possible of harmful and irritating tobacco smoke," the task force report said.

The City Council held a public hearing on the proposed ordinance in September, and several restaurant owners said they opposed it.

But council members said nonsmokers must take top priority.

"I'd like to see us go further," said council member Redella S. (Del) Pepper, a Democrat.

"A lot of people are allergic to smoke, and they deserve a break."