MOSCOW, JUNE 20 -- Uzbekistan today became the first Soviet Central Asian republic to assert its sovereignty, declaring that Uzbek legislation will henceforth take priority over Soviet laws.

The Soviet news agency Tass said that the Uzbek legislature had adopted a declaration placing "all issues of domestic and foreign policy" under its authority. Similar steps have already been taken by the Russian republic, the three Baltic republics and the three Transcaucasian republics of Azerbaijan, Georgia and Armenia.

With a population of nearly 20 million, Uzbekistan is the largest Central Asian republic, and its action is likely to set a political example for the other four Soviet republics in one of the most backward and unstable regions of the country. Nearly 300 people have been killed in ethnic disturbances in Uzbekistan and the neighboring republics of Turkmenistan and Tajikistan over the last two years. At least 78 were reported killed in rioting in neighboring Kirghizia this month.

In response to the growing pressure for greater autonomy for all 15 Soviet republics, President Mikhail Gorbachev has called for renegotiation of the 1922 treaty that established the Soviet Union. The new treaty, which could be ready by the end of this year, is likely to recognize formally the right of the republics to run their own affairs, reserving only such matters as defense, communications and foreign policy to central authorities in Moscow.

In conceding the demands for regional sovereignty, which he fiercely resisted when they were first put forward a year ago by the Baltic republics, Gorbachev is attempting to draw the line short of full independence. Estonian officials said today that Soviet authorities had threatened them with a drastic cut in oil supplies beginning next month unless they agree to freeze their moves toward independence.

The Estonian official in charge of negotiations with Moscow, Endel Lippmaa, accused the Kremlin of preparing a "creeping blockade" against Estonia and Latvia. The Soviet authorities have sharply reduced shipments of oil and natural gas to the third Baltic republic, Lithuania, for more than two months in retaliation for Lithuania's March 11 declaration of independence.

"Talks with Moscow have been hopelessly blocked," said Lippmaa, in an interview in Tallinn, the Estonian capital. "Hints that Gorbachev was ready to talk with us are just Soviet propaganda." He accused the Soviet president of a "Janus-like policy," showing a reasonable face to the West while adopting a consistently tough line toward Baltic aspirations for independence.

Estonian Foreign Minister Lennart Meri said he had been informed that the Kremlin intends to cut oil shipments to his republic from 40 billion tons to 18 billion in July, a cutback, he said, that would create severe problems for this year's harvest. Estonia, like Lithuania and Latvia, has accused Moscow of freezing its bank transactions, making it difficult to conduct normal commerce.

Gorbachev met the three Baltic presidents in Moscow last week for the first time since Lithuania's declaration of independence. But Meri said today that the three leaders were treated to a "brutal monologue."

Moves by other republics to assert their sovereignty have raised hopes in the Baltic republics of circumventing the Kremlin's economic sanctions by establishing direct trading links with other parts of the Soviet Union. So far, however, the assistance has been largely symbolic. The Georgian legislature today approved a declaration recognizing the right of the Baltic republics to self-determination and expressing a willingness to establish economic and political ties with them.

One member of the Uzbek legislature, Mohammed Saleh, said that body had considered describing its sovereignty resoluton as a "declaration of independence," but that the final document was modified on the insistence of some Uzbek leaders who wished to make it more acceptable to Moscow.