MOSCOW, JUNE 24 -- The Soviet Union is expected to release hundreds of political prisoners this year under a general amnesty declared in honor of the 70th anniversary of the Bolshevik Revolution.

Such amnesties have been declared several times to mark important Soviet anniversaries. This amnesty will be the first in Soviet history to apply to people sentenced for such political crimes as "anti-Soviet" slander and violation of laws regulating religious practices, Soviet Foreign Ministry spokesman Gennadi Gerasimov said.

Commenting today on the general amnesty, Gerasimov noted that it is "broader" than previous ones. "It should be," he said. "First, it is the 70th anniversary; second, there are new people at the top and thirdly, under glasnost, some of these crimes are now . . . " and he made a dismissive gesture with his hand.

"It has never happened before under Soviet power," said Sergei Grigoryants, a dissident writer who was released from prison last February when authorities began a formal review of political cases. So far, about 150 political prisoners have been released.

No official figures on the number of people affected by the amnesty are available, and even the number of Soviet prisoners is not publicly documented. Dissident and diplomatic sources today gave estimates of the number of people serving terms for political crimes ranging from 500 to 2,000.

Some activists cautioned today that the amnesty for political prisoners could still be held up. The June 19 amnesty decree bars amnesty for people who have violated prison or camp rules. The decree does not include prisoners confined to psychiatric wards, human rights activists here noted.

But the amnesty, combined with rising emigration of Soviet Jews, Armenians and Germans, shows increased flexibility in Soviet attitudes to the human rights issues that have most tarnished the country's image abroad.

Last month, 871 Soviet Jews left the country via Vienna.

The number of Soviet emigrants leaving for the United States, of which about 80 percent are Armenian, has also jumped dramatically, according to U.S consular officials. More than 250 cases are expected to be processed this month, compared to fewer than 20 per month in January and February and about 160 in all of 1985.

So far this year, 2,779 ethnic Germans have received permission to go to West Germany -- 992 in May alone. This compares to 417 in all of 1985. But, noted a West German official, it is still less than the peak of 10,000 a year reached in 1976.

In most respects, the amnesty, which will take effect over the next six months, applies to the same category of prisoners freed under decrees in other years, including 1977 and 1985. Older people, veterans, invalids, pregnant women and mothers of small children will be among those released, as well as people serving terms of less than three years.

This year, for the first time, statutes commonly used for political offenses were not excepted from the decree. These include article 190, which prohibits "fabrications defaming the Soviet State and public order."

Article 142, which covers separation of church and state, and has been used against people who arranged group religious instruction for their children, is also included in the amnesty.

The more serious offense of anti-Soviet agitation does not fall under the amnesty itself, although people now convicted under that statute, article 70, could have their terms reduced by one-half under the terms of the decree, according to Gerasimov.

Most of the estimated 150 political prisoners released during the first half of this year were serving sentences under article 70. According to Gerasimov, the review of political cases is continuing, independent of the amnesty.

Since then, Soviet spokesmen have disclosed an ongoing review of the criminal code, and reports of legal violations are often reported in the press. In the newspaper Moscow News today, Party Secretary Anatoly Lukyanov, was quoted as saying some constitutional rights in this country are not observed properly.

So far, nothing definitive has surfaced about the nature of legal reforms, although some have suggested that some of the "political" statutes need to be changed.

Some western diplomats have noted that Soviet authorities might feel it necessary first to free prisoners incarcerated under certain criminal statutes, before moving to amend or abolish the statutes.

Roy Medvedev, a dissident Soviet historian, predicted the amnesty will involve the largest release of prisoners in 30 years. He and Grigoryants said it is the only amnesty in Soviet history to cover political prisoners. In the 1950s, then-Soviet leader Nikita Khrushchev freed millions of prisoners from labor camps built by Joseph Stalin under a mass rehabilitation program.