The Justice Department, which has opposed court-ordered busing as a remedy for school segregation, presented its alternative yesterday for the first time. It includes a magnet school plan of "Montessori schools" in black neighborhoods and "fundamental, back-to-basics" schools in white neighborhoods.

The plan, which has not been accepted by the court or the plaintiffs, was filed by William Bradford Reynolds, head of the department's Civil Rights Division, in a federal district court in Louisiana in a case involving court-ordered school busing in East Baton Rouge, La.

Robert Williams, one of the lawyers for black students in the case, called the Justice Department plan "a blueprint for segregation . . . . I find the concept to be ludicrous . . . that the black and white community after 20 years will all of a sudden . . . desegregate the schools." He said he will oppose the plan.

Last August, the Justice Department had asked the court to reconsider the case, Clifford Eugene Davis Jr. vs. East Baton Rouge Parish School Board, et al, on the grounds that white students were fleeing the system. It was the first time the administration had asked a court to restrict a plan already in effect.

Then in September, Reynolds said he would be willing, at the request of local school districts concerned over possible white flight, to urge courts to reopen other busing cases.

Yesterday's filing, described by Justice officials as the first major indication of the administration's new policy in school desegregation cases, would generally allow students to go to any school they want to attend.

The policy change was criticized by civil rights advocates, who said the courts use busing only when other remedies do not work.

William Taylor of the Leadership Conference on Civil Rights said, "Lawyers who try these cases all recognize a role for magnet schools, but the only time magnet schools have worked effectively to bring about desegregation has been in combination with madatory reassignment to schools.

"In legal terms, the remedy for mandatory segregation is mandatory desegregation, and nowhere could that be clearer than in East Baton Rouge, where there is a long history of deliberate segregation and resistance to the Brown decree," he said, referring to Brown vs. Board of Education, the landmark case requiring school desegretation.

Taylor added, "A lot of people will object to the stereotyping of the educational needs of black students and white students."

The brief filed by Reynolds said, "The constitutional mandate to dismantle dual school systems hardly requires federal courts to preside over the demise of local systems of education." He said that in some grade levels as many as 25 percent of the white students have left the public schools since busing started in 1979.

According to the plan, which was prepared by Boston University political science professor Christine Rossell, students could go either to their neighborhood school or to another desegregated public school in the district.

Magnet schools would have special programs like computer training, foreign languages, music, art and technical career courses, according to the Justice Department plan.

The plan would provide for Montessori-type magnet schools in black neighborhoods because "whites who volunteer for magnet schools in black neighborhoods tend to be more 'liberal' and of higher social class than those who do not. At the high school level, examination schools have consistently proven attractive to such parents. . . . "

The plan added that "magnet schools in white neighborhoods should be of the kind that appeal to working class black parents (fundamental, back to basics, etc.)."

As a final incentive to encourage parents to send their children to racially mixed schools, it also provides for black principals in white neighborhoods and white principals in black neighborhoods.

Reynolds and Rossell concede that under the alternative plan some racially segregated schools are likely to continue to exist.

"There is, in candor, no guarantee of success. A critical factor in the plan's effectiveness will be the degree of support provided by the school board," Reynolds said, adding that the court should not adopt the plan if the local school board is unwilling to cooperate.