The Justice Department took the unprecedented legal step yesterday of asking a federal judge in North Carolina to overturn the 1972 state court firebombing convictions of the Wilmington 10.

Drew S. Days III, chief of the department's civil rights division, said in an 89-page brief that a review of the case uncovered evidence that the defendants were denied their consitutional rights to a fair trial. The defendants' right were violated, according to the brief, because she state prosecutor and judge suppressed a statement that raised doubts about the testimony of Allen Hall, the key prosecution witness.

Hall, then 18, was the only witness who placed all the defendants - nine black men and one white woman - at the scene of the firebombing of a white grocery store during a racial disorder in Wilmington in 1971.

Only one of those convicted, Benjamin F. Chavis, is still prison, but the others are still under state authority because of paroles.

The so-called Wilmington 10 case has attracted worldwide attention because supporters have charged that the defendants were imprisoned because of their political beliefs. Anmesty International has called the group the leading example of political prisoners in American. Soviet leaders point ot the case as a symbol of injustice in the United States.

The consitutional issues cited yesterday by the Justice Department have been raised before by defense attorneys in appeals through the state court system.

James E. Ferguson II, of Charlotte, the chief defense lawyer, said yesterday "the significance is that the Justice Department has added its weight to our position."

Richard N. League, assistant attorney general in North Carolina, who handled the case for the state, said the constitutional arguments already had been rejected in previous appeals.

League also said the Justice Department brief seemed intent on "inflaming public opinion" when it raised ethical questions about his private contacts with the state judge during post-conviction hearings.

The cumulative effect of the irregularities in the case warrant a fresh look by U.S. District Court Judge Franklin T. Dupree, the brief suggested.

The defendants asked Dupree to grant a "writ of habeas corpus," freeing Chavis and ending parole restrictions for the others.

Such requests are commonly made by prisoners convicted in state courts. This is believed to be the first time, however, that the Justice Department has asked to enter such a case as a "friend of the court."

A department spokesman said the action was taken because Justice officials had some expertise in the case, from earlier grand jury investigation, that they thought should be passed on to the couret.

A federal grand jury last year investigated charges by Hall that he testified falsely in the trial because of threats and promises of leniency by the prosecutor, James T. Stroud Jr. The grand jury returned no indictments.

The Justice Department continued its review of the case early this year after North Carolina Gov. James B. Hunt Jr. refused to pardon the defendants. It was during this most recent review that Justice attorneys apparently found a copy of an amended statement Hall had given to prosecutor Stroud before the original trial.

That amended statement was not turned over to defense attorneys during the trial despite repeated requests. The Justice brief said "since there is a reasonable likelihood that the jury's verdict might have been different" if that statement had been probed at the trial, "one would conclude that the trial was imbued with a fundamental unfairness and was in violation of the due process clause of the Fifth Amendment."

If Judge Dupree declines to free Chavis, he should at least hold a hearing to hear defense attorney arguments about irregularities in the state prosectuion, the Justice Department brief argued.

The case is "unique" the brief added, because Hall and two other key witnesses have recanted their testimony, there is evidence their testimony was untruthful, and each acknowledged getting special treatment and some inducement for their testimony.

Stroud denied there were irregularities at the trial, according to United Press International.

Despite the sensitivity of the federal intervention in the case, the White House was informed of the Justice Deparment's action only shortly before the brief was filed, a department spokesman said.

Ann Shephard Turner, one of those convicted in the case, said Yesterday: "This is what we have worked so hard for. We knew if we just kept pressing there had to be justice somewhere in all this craziness over the years."