ANNAPOLIS, Md. (Reuters) - Prosecutors on Thursday urged Maryland's high court to force a Baltimore policeman to testify against his fellow officers in connection with the death of Freddie Gray, a black man who suffered a fatal injury while in custody.

Assistant Attorney General Carrie Williams told a seven-judge panel at the state Court of Appeals that Officer William Porter would have limited immunity, barring prosecutors from using his words against him at his retrial in June.

She said the state had the burden of proving no evidence against Porter was derived from his testimony in other trials.

But Gary Proctor, a lawyer for Porter, said his constitutional right against self-incrimination would be compromised despite the immunity.

"The only way to effectively ensure that Officer Porter's rights are protected is to keep him off the stand when he has a pending manslaughter case," said Proctor, as Porter looked on intently in a packed courtroom.

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Porter and the other five officers face charges after Gray, 25, suffered a fatal neck injury while being transported in a police van on April 12. He died a week later, setting off protests and rioting.

Gray's death added to protests over police killings of black men in cities including New York, Cleveland and Ferguson, Missouri.

Prosecutors tried Porter first, hoping to secure a conviction before using him as a witness against other officers. But his case ended in a mistrial after a jury could not reach a verdict in December.

Porter testified in his own defense with prosecutors arguing at trial that he had lied. That, Proctor said, made it especially problematic that the state now intends to call him as a witness.

"The state has already labeled what he is going to say perjury," he said.

But Williams said prosecutors will not elicit any testimony they believe is false.

"This is a unique situation," said Court of Appeals Judge Lynne Battaglia. "It's not like any other situation we have seen."

Baltimore City Circuit Court Judge Barry Williams, who is overseeing the cases, previously handed each side a partial victory on the issue.

He ruled Porter, 26, must take the stand against Officer Caesar Goodson and Sergeant Alicia White.

But the judge also denied the state's motion to force Porter to testify against the other three officers - Lieutenant Brian Rice and Officers Garrett Miller and Edward Nero.

State prosecutor Michael Schatzow told the appeals court that Williams improperly substituted his judgment for the government when he denied the second request.

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But defense lawyers argued that the judge had properly rejected the prosecutors' motion as little more than a stall tactic.

Douglas Colbert, a law professor at the University of Maryland who attended the hearing and has followed the case closely, said he would be surprised if the defense prevailed.

"The prosecution has the stronger of the legal arguments unless this court wants to change Maryland law," he said.

The five officers' trials are on hold pending a decision from the court, for which there is no timetable.

(Writing by Joseph Ax; Editing by Scott Malone and Bill Trott)