A D.C. Superior Court judge declared unconstitutional yesterday a prohibition against protests on the Supreme Court plaza and steps when court is not in session and acquitted an abortion protester on charges of demonstrating there.

While lawyers for the protester hailed the ruling as an expansion of First Amendment rights to demonstrate at the Supreme Court, U.S. Attorney Joseph E. diGenova said the ruling will not affect other cases brought under the statute, which police and his office will continue to enforce.

The ruling by Superior Court judges, as opposed to rulings by appeals courts, are not binding on other judges, diGenova said, adding, "The law is still on the books and it will be enforced."

Arthur Spitzer, legal director of the American Civil Liberties Union in Washington, said the decision "was a courageous one and a step forward for the First Amendment." But he agreed that he would still have to advise prospective protesters that "for the time being, the government is still free to continue prosecuting."

The decision yesterday by Judge Donald S. Smith came in the case of Joseph P. Wall, a Philadelphia man who was among 29 persons arrested at a demonstration on the Supreme Court steps Jan. 22. The demonstration was part of a massive protest to mark the 12th anniversary of the controversial Supreme Court decision legalizing abortion.

Many of the demonstrators marched from the Ellipse to the Supreme Court, and others, including Wall, joined them on the steps. Wall, a 57-year-old auditor for the city of Philadelphia, said he knelt there with the others, praying loudly "for the Supreme Court justices to have a change of heart" on the abortion issue.

Twenty-nine protesters were arrested after kneeling and refusing to disperse. Twenty-eight pleaded guilty to the charges, but Wall pleaded not guilty and went to trial yesterday, according to his attorney, J. Andrew Chopivsky.

The protesters were charged under a federal law, enacted in 1949, that prohibited demonstrations in the building and on the Supreme Court grounds, including the sidewalks on its perimeter. In 1983, the high court ruled it was unconstitutional to ban protests on the sidewalks, which are considered public forums for the purpose of First Amendment activities. The high court, however, refused to strike down the entire statute.

Yesterday, Smith expanded on the Supreme Court's ruling by finding that the plaza and steps are also public forums, places open to the public where people are free to come and go. Banning protests there -- when the court is not in session -- infringes on a defendant's First Amendment right to freely express himself, Smith ruled.

He also asserted that he was not addressing the question of whether that ban is constitutional when the court is in session.

A spokesman for the Supreme Court refused to comment on the ruling yesterday.

John Cavanaugh-O'Keefe, of the Washington-based Human Life International and one of the protest leaders, said the 28 protesters who pleaded guilty were sentenced to one day in jail, and then given credit for the day they spent in jail after their arrests.

The ACLU's Spitzer said the significance of Smith's ruling would depend on how an appeal is decided by higher courts.

But diGenova said that because the judge ruled while finding Wall not guilty of the misdemeanor charge against him, the government cannot appeal the case. Prosecutors may not appeal when a defendant is acquitted of criminal charges