Soviet authorities today released Josef Mendelevich, the last remaining imprisoned Jewish member of a group that in 1970 attempted to hijack an airliner in Leningrad and fly it to freedom.

News of the release, which is seen here as a gesture from the Brezhnev Politburo to the West and especially to the hard-line Reagan administration, came from Jewish relief sources in Vienna shortly before Mendelevich arrived there from Moscow.

Mendelevich's departure from the Soviet Union was one of the final stages in a journey that began for Mendelevich and 11 others more than a decade ago, when few Soviet Jews were allowed to leave the country. The 12, aided by more than a dozen other people, including two gentiles who also wanted to emigrate, tried to hijack a Soviet AN2 biplane from Leningrad's Smolnyi airport and fly it to Sweden. The group, which included a pilot to fly the biplane, got as far as the waiting line when KGB police arrested them.

The subsequent trial of the Leningrad hijackers became a cause celebre in the West and among Jewish support groups.Mendelevich originally was sentenced to 15 years, but an appeals court shortened his term to 12, in clear reaction to world opinion. Mendelevich spent the next decade in strict regimen labor camps, frequently subsisting on dried bread and water because the camps refused to allow him to observe kosher food laws.

All the Jewish members of the group have been released from prison and from the Soviet Union. In 1979, the Kremlin leadership on two different occasions freed a total of seven of the would-be hijackers -- two of them as part of a swap for two convinced Soviet spies, and the others in a gesture to impress some visiting U.S. congressmen.

The two non-Jews of the group are still in prison. They are Yuri Feodorov, whose sentence ends in 1985, and Alexei Murzhenko, whose term ends in 1984.

The Leningrad trials cast a dramatic spotlight on Soviet emigration policies. Since then, Soviet authorities, in attempts to improve their image and gain better trade ties with capitalist countries, have allowed more than 200,000 of the estimated 3 million Soviet Jews to leave the country.

The release of Mendelevich comes during a new spurt in Jewish emigration here. Last year, about 21,000 Jews were allowed to leave, well below the 1979 peak of more than 50,000.

Analysts believe that in the past few weeks, as many as 300 families have been cleared to leave. There are reports that as many as 150 Jews a day are now receiving exit permissions, although these figures can only be accurately proven in Vienna, the principal clearing point for Soviet Jews leaving the country.

In recent times, it has been customary for the Kremlin under Leonid Brezhnev to make some gestures toward the West on the eve of Party Congresses; a party conclave begins next week. Such moves are aimed both at reducing the number of discontented people in the capital during the congress and at projecting a better image to the world. Mendelevich's surprise release seems aimed at both objectives.