Some 75,000 Northern Virginia residents could lose free legal services if Congress approves massive cuts in legal aid for the poor, officials of the Legal Services of Northern Virginia said this week.

President Reagan has called for elimination of the $321 million program that finances legal aid for the poor and has asked for abolition of the independent corporation that oversees it.

"It would be almost impossible for us to continue without federal funding," said Charles Vasaly, executive director of the Northern Virginia program, which depends on federal funds for nearly two-thirds of its support. "We knew when Reagan was elected that he was out to kill us, but we didn't know it would be this bad.

"It's a travesty (to cut funds). Legal Services provides a day in court for someone who would otherwise not have access to justice."

Legal services provides counsel on civil matters, Vasaly said, and handles no criminal cases.

Long a thorn in Reagan's side, Legal Services Corporation -- the national parent body that administers and funds local programs -- would be eliminated under Reagan's proposals and its work performed at the discretion of individual states. However, earlier this month the Senate Committee on Labor and Human Resources voted to restore $100 million for legal aid to the poor, to be administered under a block grant. A committee spokesman said no decision has been made about whether the $100 million would be earmarked for legal aid services or if states could use the money for other purposes.

Locally, Legal Services of Northern Virginia employs 15 full-time lawyers and 10 paralegals and receives more than $500,000 in federal money to finance its annual budget of $760,000. The rest of the funds come from local governments. According to Vasaly, more than 75,000 Northern Virginians are eligible for free legal aid.

Vasaly said the loss of federal funds could not be made up by local governments. In fact, some area governments already are considering cutting back their support for the program. In Alexandria, for example, City Manager Douglas Harman has recommended a 20 percent decrease in city funds for the program for next year.

The effect of federal and local cuts, Vasaly said, would be devastating. The Northern Virginia program recently consolidated several legal services groups -- serving Fairfax, Arlington, Prince William, Loudoun and Alexandria -- and additional attorneys have been hired to staff new offices in the area. With the funding cuts, satellite offices in areas heavily populated by low-income residents, considered central to the effectiveness of the program, would be closed and support staff could no longer be paid, Vasaly said.

Critics of legal aid complain that programs have lost sight of their function and should be abolished.

"The Legal Services Corporation is a $300 million annual government subsidy of a leftist ideological agenda," said Howard Phillips, national director of the Conservative Caucus, a group long opposed to the corporation.Phillips accused the corporation of spending the majority of its time fighting class-action abortion and busing cases and ignoring its mandate to concentrate on individual civil cases.

Vasaly, however, said most Legal Services cases involve problems that individuals face, such as family disputes or consumer complaints. Three attorneys and four paralegals, he said, work only with the elderly.

Of the nearly 1,000 cases handled by the Northern Virginia group since last July, Vasaly said, 36 percent involved family disputes -- divorce, alimony, child custody -- and more than 20 percent involved individual consumer complaints. Vasaly conceded that lawyers in the program had handled some class-action suits -- most notably an Arlington County case involving eligibility requirements for food stamps -- but that those cases are few.

"The majority of cases that we handle are routine, very routine. But they are very important day-to-day matters that affect people who line on the margin," Vasaly said.

"Is it fair to suggest that because we represent someone who is poor, that we should do a lesser job of advocating than we would for someone who is rich? We just want to give them a day in court."