Twelve abortion protesters were sentenced to one day in jail and fined $10 yesterday after pleading guilty to illegally demonstrating Tuesday on the steps of the Supreme Court Building.

All were released on one year unsupervised probation after a D.C. Superior Court hearing commissioner gave them credit for the day they spent in jail after their arrests.

They were among 29 persons arrested at a massive demonstration that followed a march from the Ellipse to mark the 12th anniversary of the high court's decision legalizing abortion. At least five others have said they will also plead guilty to the charge while four of the demonstrators have vowed to go to trial.

Seven other protesters are scheduled to appear in court next week for arraignment.

Controversy continued to swirl around the decision of U.S. Attorney Joseph E. diGenova to prosecute the abortion protesters while charges against antiapartheid demonstrators arrested on an almost daily basis outside the South African Embassy are routinely dropped.

Leaders of the antiabortion movement assert their followers are being persecuted, while those arrested outside the South African Embassy contend that they have been denied their day in court.

Some defense attorneys contend that diGenova's actions raise questions about whether his charging policies are discriminatory, but legal experts said prosecutors have almost unlimited latitude in deciding which cases they want to take to court.

"What the courts are likely to say is that this is the essence of prosecutorial discretion," said lawyer Sebastian K.D. Graber, a specialist in First Amendment issues who has challenged the U.S. attorney's charging policies in the past.

"There is a fine line between what is legitimate discretion and what is selective prosecution," Graber said. "It may be that the courts will have to draw that line."

Graber said that while the law forbids vindictive or discriminatory prosecutions, discrimination is difficult to prove. Graber is appealing the case of a protester arrested for camping in Lafayette Square across from the White House. The man should not have been prosecuted, Graber contended at trial, because Vietnam veterans were not charged for staging a protest by camping on the Mall.

"It raises just a general equal protection question," Graber said. "One class of protesters is treated one way, whereas others are being prosecuted with impunity even though the issue involved is just as important."

DiGenova was in Baltimore yesterday helping the prosecution of a man accused of bombing local abortion clinics, and could not be reached. Officials in his office continued to maintain a strict silence on the decision to prosecute the antiabortion demonstrators.

One source close to the office defended the charging decisions, saying the government has a much stronger interest in preventing demonstrations near the nation's highest court than in keeping protesters away from one of many embassies in the city.

The source noted that justices have expressed a keen interest in maintaining the integrity of the court grounds, while South African Embassy officials declined to press charges against the demonstrators there.