A group of American civil libertarians concerned with the condition of Soviet Jews convened an extraordinary tribunal on Capitol Hill yesterday. Its purpose was to gather evidence for the defense of an imprisoned Russian Jew.

The tribunal had all the trappings of an official inpuiry, and witnesses were sworn in by Chief Judge David Bazelon of the U.S. Court of Appeals for the District of Columbia.

But for all its official gloss, the tribunal was a novel public relations effort designed to demonstrate American public support for Anatoly Scharansky, 29, a computer expert who was denied a visa to emigrate to Israel and who was arrested in Moscow seven months ago on treason charges.

A coalition of Jewish activists and civil libertarians in the areas of politics, universities, foundations and law organized the tribunal.

It included such persons as Columbia University President William J. McGill; Sen. Frank Church (D-Idaho); Bayard Rustin, president of the A. Phillip Randolph Institute; Robert McKay, derector of the Aspen Institute for Humanistic studies, and Chesterfield Smith, former president of the American Bar Association.

According to Alan Dershowitz, a Harvard law professor and counsel for Scharansky, the goal of the tribunal was to prepare a brief for Scharansky's proceedings will be sent to Roman Rudenko, the chief Soviet prosecutor.

Scharansky was arrested in March after the Soviet government newspaper Izvestia accused him of working for the Central Intelligence Agency. He had applied in 1973 to go to Israel, but was turned down on grounds that his training ascomputer expert had given him a access to state secrets.

Since 1973, he has gradually emerged as one of the boldest and most articulate dissident activists. Because of his command of English, he had frequent contacts with American journalists in Moscow and served as an informal spokesman for other Jewish "refusedniks," as those who had been refused permission to emigrate are called.

His arrest and the subsequent interrogations of persons connected with him has suggested that the Russians may be preparing a spectacular show trial in which U.S. diplomats and journalists in Moscow would be accused of recruiting dissidents for CIA activities.

The thrust of tribunal inquiries yesterday was to establish that Scharansky had conducted his activities publicly, that his objective was to publicize the plight of Soviet Jewry, that he had no connection with the CIA and that his computer research had focused on ways to program computers to play chess.

Among those testifying in Scharansky's behalf were his wife, Nataliya, who lives in Israel; Isaak Elkind, a Soviet attorney who also emigrated to Israel; Jack Minker, professor of computer science at the University of Maryland, and two Americans who had contacts with Scharansky in Moscow. Rep. Robert F. Drinan (D-Mass.) and Alfred J. Friendly Jr., who served as Newsweek correspondent in Moscow from 1974 to 1976.

Also introduced into the record was President Carter's statement last June that an investigation had revealed no CIA contacts of any kind with Scharansky.

Friendly summed up the tone of yesterday's testimony by saying that "Scharansky's only crime was to speak the truth, and worse of all to do so in English."

McGill, the tribunal chairman, had earlier asked Soviet Ambassador Anatoliy F. Dobrynin to send representatives to the proceedings, but theinvitation was ignored