The Justice Department urged a federal appeals court today to allow the Norfolk school system to dismantle its 14-year-old court-ordered busing plan in a case that could have dramatic ramifications for school desegregation programs nationally.

The 22 black plaintiffs opposing the Norfolk School Board's proposal to end cross-town busing of elementary pupils said that court endorsement of the plan could unravel 30 years of efforts to integrate and equalize the nation's schools.

"It would permit school districts across the South and the North to resegregate schools despite . . . 30 years of litigation," Julius Chambers, head of the Legal Defense Fund of the NAACP, told the three-member panel of the 4th Circuit Court of Appeals here.

Assistant Attorney General William Bradford Reynolds, head of the Justice Department's civil rights division, argued that the case represents a major turning point in the history of school desegregation. At issue is whether school systems that have eliminated "separate but equal" schools should be allowed to discontinue busing and maintain neighborhood schools, no matter what their racial composition.

It is the first time Justice has intervened in a lawsuit addressing the question of ending court-ordered busing after a school system has complied with all court-ordered desegregation mandates.

The Justice Department involvement in this case appears to be a sign of growing opposition to school busing from the Reagan administration. Although the administration has consistently refused to support busing plans in new desegregation cases, this is the first time it has endorsed abandoning previously ordered busing plans.

The one-hour legal duel before the appeals panel and a packed courtroom pitted Reynolds against one of Virginia's best-known civil rights lawyers, Henry L. Marsh III.

Marsh, the first black mayor of Richmond and the lawyer who argued the original 1971 discrimination suit against the Norfolk School Board, labeled the board's plan to abandon cross-city busing for elementary students "invidious and race-based." He said the board members "have catered to the wishes of these white parents."

Reynolds argued that Norfolk's new neighborhood-based school proposal is "a perfectly constitutional plan and there is no reason for the courts to intrude."

Because the Norfolk school system has complied fully with desegregation orders, Reynolds said, it is "entitled to be treated just as any other unitary system."

"You can't be serious in saying that we can't judge the background of all this," Judge James M. Sprouse shot back. "That just because you have a judicial decree that you should close your eyes to discrimination prior to 1971?"

"If it has remedied the wrong," replied Reynolds. " . . . It's not enough to simply look at the numbers, you have to look at whether there's intentional discrimination."

But, Marsh responded, "It is clear the 1983 plan simply continues this board's eternal quest to operate segregated public schools." He accused the board of trying to "create a bunch of predominantly white schools."

The Norfolk School Board voted two years ago to stop cross-town busing of elementary school pupils in an effort to halt the continuing white flight from the inner city to the suburbs and to slow the declining school enrollment. The board said white flight threatened to leave Norfolk with many virtually all-black schools, a problem faced by many urban areas across the country.

The school board's proposal, approved last July by U.S. District Judge John MacKenzie, would create a system of neighborhood elementary schools. Ten of them would be more than 95 percent black; one would be more than 70 percent white and 26 schools equally balanced racially, according to school officials. Currently, no school is more than 90 percent black, one school is 73 percent white and the others have racial ratios of approximately 60-40 and 50-50.

Officials said in court that Norfolk's percentage of black pupils in elementary schools increased about 10 percent between 1971 and 1983.

Norfolk school attorney Jack Greer argued that while the student ratios could return to those found before compliance with desegregation orders, the situation in the schools has changed dramatically to a system that now provides equal education to blacks and whites. He said the court-ordered "remedy has become counterproductive."

"It perpetuates the effects of past racial discrimination," Marsh contended. He accused the school board of contributing to the concept of segregated schools when it agreed to place 22 of its schools in the midst of each of the city's government-assisted housing projects, all but one of which are "all black," he said.

Judges on the panel included Sprouse of West Virginia, H. Emory Widener Jr. of Virginia and Sam J. Ervin III of North Carolina.