When Virginia's welfare department last year attempted to deny fuel assistance funds to some low-income families, it encountered opposition from an unexpected source.

The federally funded, Arlington-based Legal Services of Northern Virginia Inc., which has maintained a low profile and prefers a low-key approach to litigation, surprised everyone by filing a class action lawsuit against the state that eventually forced the officials to back down.

It was a rare action by an agency that has studiously avoided the class action lawsuit approach that has made many other legal services agencies, all of them set up to defend the rights of the poor, highly controversial. Only twice in the past two years has a branch of the Northern Virginia agency filed such suits.

"One of the allegations that has been made against legal services agencies is that we file, willy-nilly, class actions," said Charles E. K. Vasaly, executive director of the Northern Virginia agency. That, he said, has given legal service agencies a reputation for "tilting, fighting, knocking the establishment, trying to upset whatever the good social order might be by using this mechanism, going into federal court, hassling the establishment."

That approach, however, is not Vasaly's. Instead, he has insisted that the agency's staff of 16 lawyers, working out of six offices in the Washington suburbs, concentrate their efforts on specific problems such as divorce, social security, consumer, welfare, housing, tenant-landlord and employment actions on behalf of indigent clients.

By law, federally funded legal aid agencies can handle only civil cases and must provide their services free to clients who meet strict income eligibility standards. In Fairfax and Arlington Counties and in Alexandria, the maximum income eligibility cutoffs are slightly higher than the national standard because the localities have provided funds to the Northern Virginia agency to offset the high cost of living in the area.

Still, the agency's caseload--particularly domestic complaints--is so overwhelming that local private bar associations have been asked to help ease the burden. In return for their taking on pro bono cases, the agency refers fee-generating cases to private lawyers.

Last year, Legal Services' workload totaled almost 3,700 cases, or roughly 6 percent of the 23,000 legal aid cases handled statewide.

Some examples of successful agency efforts last year:

* In Arlington, a disabled natural mother regained custody of her young son after relatives, who had taken the boy to visit them, refused to return him.

* In Alexandria, a client who couldn't afford an attorney won $30,000 in retroactive Social Security benefits..

* In Fairfax County, a man who lost his job after suffering a stroke was sued by a bank for $500 in back payments. The bank seized his car and was ready to sell it when the agency went into action. The man not only ended up keeping his car, but Legal Services also arranged a plan for the man to pay off his debt to the bank.

* In Prince William County, a woman and her three small children were threatened with loss of their gas service until the agency proved the landlord had deliberately failed to deliver gas bills to her. A court blocked the shutoff.

Many attorneys regard Legal Services with skepticism, however, although they seem to agree that the agency provides valuable and needed services.

Joanne Alper, an Arlington lawyer who is president of the county's bar association, said there is some suspicion among private practitioners that legal services lawyers are taking some cases that they should be getting--cases that could produce fees.

"It seems to me that, in some domestic relations cases and Social Security claims, the private bar could handle more of these," Alper said. "But the problem I can perceive is more with the perception of its being a big bureaucracy, like a super law firm. To many lawyers, there's a feeling they're in competition with it. They resent the fact their tax dollars are going in there."

Many lawyers also believe there is a duplication of effort, Alper said. "They say we're paying twice, in essence, for the same case," she said, "once through taxes and once through our own time pro bono."

Charles Geschickter, a Fairfax County attorney who is president of the bar association there, said: "I am not too happy about having a national organization come in and give services in a local jurisdiction . . . . I think we've kind of lost sight of the private bar giving service to the indigent."

But Don McDonough, the managing attorney for the agency's Fairfax County office, disagreed. "We're not shipping out less complex cases so we can handle more complex cases in-house," he said recently. "As with the rest of the economy, many private attorneys are having rough times. The last thing they want to see is a $4,000 pro bono case."

Other complaints center on whether Legal Services is truly serving the indigent, and Vasaly acknowledges that verifying a potential client's eligibility can be difficult unless the client has already been screened by a welfare agency, for example. Given time constraints that often see the agency's attorneys having to handle a case the day after they are first approached by the client, Vasaly asks, "How do you verify someone is not working?"

"This is a far, far better structure and program than the one that existed before," said Vasaly. "We are serving far more clients and the quality of service is far better than when there were several independent legal aid agencies."

Because Legal Services is able to coordinate the local branches and provide a pool of attorneys with expertise in diverse legal areas, the poor are better served, he maintained.

"Generally, people who handle cases pro bono do not handle the complex matters or problems of low-income people," Vasaly said. "They're not familiar with the Medicaid or Medicare system or the social security system, et cetera. So, on balance, I don't think you can provide those types of services from private attorneys.

"I think it would be very wrong to conclude that you don't need a core staff" at Legal Services to coordinate and advise lawyers on legal issues affecting the poor," he said.