D.C. Superior Court Judge Joseph Michael Hannon announced yesterday that he has reconsidered and will withdraw from a trial involving abortion protesters because he realized that his last-minute revelation that he had participated in the same march raised questions about the "appearance of justice."

"In my heart of hearts as a judge of the Superior Court I know I could be fair and impartial in this case . . . " said Hannon, 67, in an often emotional 20-minute speech from the bench. But, he said, "So long as I sit there are those who are going to say I favor the defendants."

Hannon announced Thursday -- shortly after the first witness had started testifying in the nonjury trial -- that he and his law clerk, John Ingram, had joined the annual March for Life parade for four blocks on Jan. 22, the anniversary of the 1973 Supreme Court decision legalizing abortion. The march ended at the Supreme Court building, where the nine defendants who were before him this week on misdemeanor charges had been arrested as they prayed on the steps.

Legal experts contacted yesterday said Hannon's earlier failure to reveal his participation in the march raised serious questions of propriety and that his decision to withdraw from the case threatened the government's ability to retry the case. One defense attorney said he will seek dismissal of the case on ground of double jeopardy.

The National Abortion Rights Action League filed a complaint yesterday with the D.C. Commission on Judicial Disabilities and Tenure, seeking Hannon's removal from the bench. The commission investigates allegations of misconduct by local judges.

Commission Chairman Dr. Bette L. Catoe, asked if Hannon's reported actions raised questions in her mind, said, "Speaking as a lay person, yes. As chairperson of the commission, I can only say 'no comment.' I'd rather not do anything at all to be unfair."

Hannon continued to hear prosecution witnesses Thursday after the prosecutor decided to withdraw her request that Hannon excuse himself from the case.

Hannon said yesterday that he decided to remove himself from the case after The Washington Post reported that the U.S. attorney's office's decision not to press for his removal was based on fears that constitutional protections against "double jeopardy" would prohibit further prosecution. "I thought the government was now telling me Judge Hannon can be fair and cautious . . . " said Hannon, his hands shaking at times as he read from notes. "But the statement in The Washington Post made by a supervisor in the U.S. attorney's office . . . carries with it a clear implication that I as trial judge in this case could not be fair and impartial."

Hannon repeated yesterday in the hushed courtroom his Thursday statement that his failure to reveal his participation before the start of the trial was an "inadvertence." Hannon said it was only after the first witness, a Supreme Court police officer, began to describe security preparations for that day that he remembered that he had joined the parade for a few blocks.

"I fault myself for not recalling my participation in the parade" earlier, Hannon said. "But I come equipped with the same inadequacies as other persons."

Hannon said the press attention and concerns by the U.S. attorney's office left him no choice but to step aside, despite a history of fairness on the bench. He pointed to his decision last week to convict 21 persons who had demonstrated outside the Soviet Embassy, saying he sympathized with their motives but had no course but to "live by the law."

Several judges and lawyers praised Hannon's reputation for fairness, although they expressed surprise at his initial reluctance to withdraw. "He may be gruff," said one lawyer, "but he's a fine, fine judge."

Hannon said the case will be assigned to another judge.

In the complaint filed yesterday with the D.C. Commission on Judicial Disabilities and Tenure, abortion rights league Executive Director Kate Michelman wrote, "Judge Hannon's failure to disclose his participation in the antichoice demonstration . . . indicates that he places his allegiance to antichoice extremists above his responsibility to the fact and the appearance of a fair and impartial judicial system."

Spokeswoman Emily Tynes said the group will pursue the complaint despite Hannon's decision to remove himself from the case because "we feel that his participation in the march itself speaks to his ability to be impartial in cases."

The seven-member commission has the power to remove a judge from office for "conduct prejudicial to the administration of justice or which brings the judicial office into disrepute." Other actions it can take range from dismissing a complaint as frivolous to issuing public or private censures or reprimands.

Specialists in judicial ethics were divided about the propriety of Hannon's joining in the demonstration but agreed that he should have revealed it.

"That's absolutely scandalous misconduct by the judge," said Yale Law School Professor Geoffrey C. Hazard Jr., who served as a consultant to the American Bar Association committee that wrote judicial ethics rules. "It was the gravest dereliction of duty to have anything to do with the case."

While Hannon "has First Amendment rights, they are not the same as other people's," Hazard said.

Northwestern University law professor Steven Lubet said it was "perfectly appropriate" for Hannon to join the march, assuming that he did it without any effort to trade on the prestige of his office. However, he said, Hannon's failure to make a timely disclosure "raises grave questions." And, he said, "Judges have an obligation not to forget things like that."

Hannon's actions raised questions about whether the defendants can be retried. The constitutional protection against double jeopardy, which generally prohibits trying a defendant twice for the same crime, is triggered in a nonjury trial when the first witness takes the oath to testify truthfully.

"That's what happened here," noted J. Andrew Chopivsky, attorney for several defendants, saying he planned to object to a new trial on that ground.

A spokesman for the U.S. attorney's office, however, said that because Hannon had found that it was a "manifest necessity" to stop the trial, the government should have no problem retrying the case.

Criminal law specialists said the government's power to continue prosecution was at best unclear. "If it's not the defendant's fault or at his request, then double jeopardy sets in and the Constitution provides that they cannot try people again. They go free," said Georgetown University Law Center Professor Paul F. Rothstein.

The defendants reacted in different ways. "It's a disgrace," said Catherine O'Connor of Rockville. O'Connor said she felt like clapping when Hannon first told of his marching. "It should be one of the proudest moments of his life."

New York native Dennis Herlihy said, "I don't want to criticize the judge, but I felt he would have had to lean over backwards . . . just to show that he was being fair.