The NAACP Legal Defense Fund today attacked Virginia's latest college desegregation plan as "illegal" and blasted the Reagan administration for approving it before the fund's lawyers could register their objections.

"It just shows how outrageous the administration's performance has been," said Joseph Rauh, attorney for the Legal Defense Fund. "The plan is illegal as hell . . . It's just a darn shame that we have in the Department of Education people whose philosophy is contrary to the enforcement of civil rights laws."

Virginia officials and the Office of Civil Rights in the U.S. Department of Education reached an agreement late Thursday on a three-year $16 million plan to integrate the state's colleges and universities. The new agreement, proposed by Gov. Charles S. Robb, revises a 1978 plan that had been declared a failure last summer.

The NAACP Legal Defense Fund is the original plaintiff in a 1972 lawsuit that precipated a series of federal ultimatums for the dismantling of once-segregated systems of higher education in Virginia and other states.

Under a court order issued in 1978, federal officials are required to give the Legal Defense Fund 72 hours to comment on any agreement reached with a particular state.

In Virginia's case this week, the fund's attorneys said they were given a copy of the proposed agreement with Virginia at 5:33 p.m. Monday. They said they sent their comments back at 5 p.m. Thursday, but that the Office of Civil Rights said they were not received until after 6 p.m. Meanwhile, about 5:30 p.m., the Virginia agreement was announced simultaneously in Richmond and Washington.

"We went forward with what we had negotiated with the Commonwealth," said Harry M. Singleton, assistant secretary for civil rights, adding that the office is now reviewing the fund's comments. "Even though the Legal Defense Fund did not comply with the 72-hour court mandate, we are still considering their comments," he said.

State officials said yesterday that they were unaware of the fund's objections when they announced the agreement Thursday.

But Rauh said today that he was outraged that the agreement was announced before the comments had even been received. "They knew it was coming," he said. "What they did showed they never had any intention of paying attention to our comments, regardless of what we told them, even if we told them four murderers were running higher education in Virginia."

The fund's main objection to the Virginia plan is that it sets goals for black freshman enrollment in 1985 that are below goals originally set for 1981. For example, in the fall of 1981, Virginia should have enrolled 5,754 black freshmen in state colleges under the old plan; the new plan calls for the enrollment of 5,477 black freshmen by 1985.

"Virginia is the first state that has said we want to do less and we want more time in which to do it," said Jean Fairfax, a fund education expert.

Virginia Education Secretary John Casteen today defended the numbers in the plan. "It is not simply a matter of saying the goals have changed but of saying that the goals are based on correct information," he said. "The question is, do you want high numbers, or do you want accurate numbers?"

Casteen had pushed for an agreement on the plan this month so the state legislature, in session until Feb. 26, could appropriate the $6.3 million required for the first year.