His face drawn into stern lines, Superior Court Judge Nicholas Stephen Nunzio stiffened visibly in his chair at the bench.

"Gentlemen, both sides," he began, "do not make a mockery of this system, not a mockery." He was talking to counsel for both the prosecution and the defense in the city's trial of 12 Hanafi Muslims on murder and kidnaping charges.

The time was 10:32 a.m. yesterday and Nunzio had just ordered the jury out of the courtroom. Once they had cleared the room, Nunzio - his patience clearly tested - turned to the lawyers and began to lecture them about what he viewed as their tendency toward mockery.

"You will face this last day with dignity," he asserted. And if they chose otherwise, he said, he would hold them in contempt of court. Publicly, he said, and not privately in bench conference.

This was Nunzio, the disciplinarian, the judge who stands firmly against the informality that is the rule in some courtrooms.

As it happened, later developments made it clear that yesterday was not to be last day of the trial. Nunzio took this development with equanimity.

By reputation, he is a law and order man who lives by old-fashioned values, and a judge who can be tough when it comes time to impose sentences. But lawyers who know him for humanity who can be swayed under the right circumstances by a plea for leniency.

Now seven weeks old, the trial has thrust Nunzio, a Superior Court judge since 1970, into a position of prominence to which he is generally unaccustomed. On his secretary's desk is a scrapbook thick with newspaper clippings, vents in his courtroom are a fixture on the nightly television news.

Nunzio knows he is being watched not only by the press and the public, but by his colleagues at the Superior Court, a scrutiny that is intensified by the presence of Harry Toussaint Alexander, a former Superior Court judge turned defense attorney in this case.

Alexander represents Hamaas Abdul Khaalis, the alleged leader of the takeover of three Washington buildings last March, during which 149 persons were held hostage and one man was killed.

The potential clash of Nunzio's temperament with Alexander's flamboyant personality concerns lawyers watching the Hanafi trial who worry that an emotional explosion could lead to a mistake and a mistrial.

When tempers flare, though, Nunzio will clear the courtroom and take a few moments off the bench to smoke a cigarette, to banter with courtroom officials, and unwind.

Two recesses are planned for each day. At lunchtime, his routine has been to set aside some time for a walk before returning to the courtroom.

Nunzio, who declined to be interviewed for this article, has cited Alexander once for contempt. But the rising pressures in Courtroom 11 have not centered on Alexander alone. Two other defense attorneys have been cited for contempt, and on several occasions during the trial Nunzio has given verbal lashings to both prosecutors and defense attorneys.

Some observors believe, however, that contempt citations are Nunzio's that it is unlikely that any contempt way of demonstrating that he intends to keep tight control over the trial, which he has said he intends to run "by the book." And it is understood penalties will be levied - unless extremely unseemly conduct occurs.

"Nick is basically a humanistic person . . . with very basic standards, moral and religious," one lawyer said of Nunzio, who is the father of six children and a Roman Catholic.

Born in 1929 in Bethlehem, Pa., educated in parochial schools, Nunzio enlisted in the Army when he was 18. He served in Korea and Germany, won the Bronze Star, married in 1953 and resigned his captaincy the following year.

He enrolled at George Washington University in 1954 and three years later graduated with distinction in foreign affairs and was elected to Phi Beta Kappa. The he attended the GW law school.

He joined the D.C. Corporation Counsel' office as a trial attorney in the early 1960s. Five years later he became an assistant in the U.S. Attorney's Office here.

As a government prosecutor, Nunzio was "a very square shooter, you could talk to him," said one attorney.

"Before he went to court, he was really fair, he didn't play really tricky games . . . But once you got to court, (he) fought like a tiger," another lawyer said.

As an attorney, Nunzio grew up on the government's side.

"There asre a lot of judges who are clearly government-minded," said David C. Niblack, codirector of the legal intern program at Georgetown Law Center, who has known Nunzio since 1965.

With Nunzio, Niblack said "if someone does something dad, he's outraged . . . and he hears terrible things in court and gets mad . . . But if you've got something on the government . . . he'll listen, wheras a lot of judges won't.

It was in part Nunzio's Italian heritage that brought him out of the U.S. Attorney Office and onto the Superior Court bench.

As the nominees were about to be selevted for judgeships on the expanded Court of General Sessions, which was to become the D.C Superior Court, then Secretary of Transportation John A. Sirica and asked him to put some Italian names on the list.

Sirica submitted Nunzio's name. In September, 1970, Nunzio was nominated to the court by former President Nixon and assumed office in October. A Democrat when he was appointed, Nunzio has since become a Republican.

On the walls of his red-carpeted chambers, along with photographs of his children and the seascapes that Nunzio has painted in his spare time, hang portraits of Nixon, Sirica and Volpe.

Outside the courtroom, Nunzio is described by lawyers as an affable, talkative, entertaining man, a toast-master without peer, a person who greets an acquaintance on the street with the enthusiasm most people reserve for a long-time friend.

As a judge, said David Niblack, "he is human . . . a judge (doesn't have) to be gray and cold and not react to any-thing . . ."