The Soviet Union, reversing its practice of the last 10 years, has announced it will not permit packages of matzoh, the unleavened bread central in the celebration of Passover, to enter the country via parcel post.

The action, disclosed this week by the National Conference on Soviet Jewry in New York is expected to hamper Passover observances of thousands of Soviet Jews. It also renews an area of tension between Soviet authorities and Jews in America and western Europe that has been quiet for the past decade.

The Paris office of the American Jewish Committee was preparing to ship 25,000 individual packages of matzoh to Jews in Russia for celebration of the eight-day holiday, which begins on April 2.

Soviet action took the form of a notice to the International Bureau of the Universal Postal Union, stating that import of products of "flour converted into bread" is forbidden.

Officials of the National Conference on Soviet Jewry said that while Jewish communities in places like Moscow, leningrad and Kiev are expected to have extra flour and facilities for baking matzoh, Jews in smaller Soviet cities would not be thus served.

The Jewish agency has appealed to the U.S. State Department and the Universal Postat Union to intervene with the Soviets in the matter, which they said violated the Helsinki accord.