Indian activist Leonard Peltier, championed by the Soviets as an "American Sakharov," was back in federal court today trying to overturn his conviction in the 1975 murder of two FBI agents.

With an observer from Amnesty International in the courtroom and a phalanx of supporters outside marching behind a ceremonial totem pole, Peltier's lawyer, William Kunstler, asked for a new trial on grounds that the FBI had deliberately concealed evidence that cast doubt on its case.

Peltier, now 40, was sentenced to two life terms following a 1975 shootout in South Dakota at Pine Ridge Reservation.

He has always maintained his innocence.

His case has become a cause celebre among European liberals and in the Soviet press, which cites it in response to western criticism of the jailing and exile of Soviet dissident Andrei Sakharov.

Whatever the European interest, today's proceeding, an evidentiary hearing ordered after the Leonard Peltier Defense Committee found new government documents, seemed to be a classically American tableau.

Presiding in an amiable, informal fashion was U.S. District Court Judge Paul Benson, a white-haired jurist who exactly fits Hollywood's stereotype of a judge.

To his left were Peltier, a Chippewa-Sioux with long black hair and a bushy mustache, Kunstler and two other long-haired defense lawyers.

Across the room sat the balding, sarcastic prosecutor, practically swimming in his overflowing files on the nine-year-old case.

In the audience were two dozen reporters and a hundred Peltier supporters, many in Indian dress. Carefully watching the Indians were two dozen finicky federal marshalls, walkie-talkies in hand and guard dogs at the leash.

Kunstler, appearing grayer and mellower than in his days as a flamboyant "new left" lawyer, raised the Sakharov parallel in his first words to the court this morning. He said later that he welcomes the Soviet support because "very few defendants get the benefit of international prodding."

"The Russians are using this for their own purposes, just like the Americans use the Sakharov case," he said. "If we're going to call the street outside the Soviet Embassy in Washington 'Sakharov Square', maybe we'll get a 'Peltier Plaza' in Moscow."

The prosecutor, Assistant U.S. Attorney Lynn Crooks, ridiculed the notion:

"I don't remember this guy Sakharov getting in a firefight where two humans were killed," Crooks said. "I don't remember anything about a Sakharov trial, or appeal, or rehearing, or habeas corpus proceedings."

The legal issue today was whether the government had honestly complied with rules of criminal procedure requiring prosecutors to show the defense any information that might prove exculpatory.

At Peltier's jury trial in 1977, the prosecutor told Judge Benson:

"There has been literally total disclosure in this case."

But when Peltier's lawyers filed a request under the Freedom of Information Act after the conviction, they received thousands of pages of FBI files they had not been shown during the trial.

Kunstler argued today that one previously released FBI teletype disputes government evidence that Peltier's gun fired the bullet that killed one of the agents.

In cross-examining FBI firearms expert Evan Hodge, Kunstler suggested that "the government had a theory" about Peltier's guilt and that laboratory results were explained so as to comply with the theory.

"That wasn't a government theory, Mr. Kunstler," Hodge replied. "That happens to be the truth."

During the hearing it was disclosed that the federal government, through the Criminal Justice Act, has financed much of Peltier's defense over the years because of his indigent status.

The government also paid for Peltier's ballistics expert at today's hearing. Earlier, the FBI loaned microscopes and other equipment to the defense lawyers so they could develop an independent ballistics explanation of the case.