Soviet President Leonid Brezhnev died here yesterday at age 75, it was officially announced this morning. He had been in power for 18 years.

The announcement was issued in the name of the Soviet Central Committee, the Presidium of the Supreme Soviet and the Soviet government. It said he died suddenly and gave the time of death as 8:30 a.m. (12:30 a.m EST) Wednesday.

The announcement hailed Brezhnev as a "true continuer of Lenin's great cause and an ardent champion of peace and communism," saying his name "will live forever in the hearts of the Soviet people an all progressive mankind."

Soviet television showed Brezhnev's portrait edged in black and the announcer read the official announcement twice.

There was no immmediate indication whether his successor already had been chosen during a meeting of the leadership yesterday.

The main candidates for the post have been considered to be Konstantin Chernenko and former secret police chief Yuri Andropov, both senior figures in the leadership.

Rumors that a senior Soviet political figure had died swept Moscow last night after Soviet television unexpectedly changed its scheduled programs and the state radio began playing classical music.

Moreover his customary signature failed to appear under official greetings sent to Angolan President Jose Eduardo dos Santos and published in the party newspaper Pravda. Brezhnev, who had last appeared in public last Sunday during celebrations marking the 65th anniversary of the October Revolution, had been in declining health for some time. But on Sunday he had appeared vigorous.

Brezhnev stayed in power longer than any Soviet Communist Party chief except Joseph Stalin.

Hardly the most innovative or domineering of modern Russian leaders, Brezhnev nonetheless achieved enormous stature, presiding at the Kremlin in a period when Russia's strength and influence around the globe reached the greatest point in its history.

Brezhnev possessed great political, tactical and managerial talents that served him well in building his career and suited the needs of the Soviet Union as it developed into a fullfledged superpower.

But for a man of as much determination as Brezhnev, the past few years must have been a time of frustration and disappointment. Abroad, his government is bogged down in Afghanistan and challenged in Poland. Detente and arms control agreements have lost appeal.

Indeed, there is some evidence that Brezhnev would have liked his legacy to be cast in terms of his contribution to peace.

Brezhnev plainly understood the advantages of detente. Good relations with the West meant access to technology and finished goods that the Soviets so badly need.

Perhaps the high point of his pursuit of detente came in 1975 with the signing of the Helsinki declaration on security and cooperation in Europe and human rights.

Brezhnev had worked for years to get the 35-nation conference convened, although in the end the final document was a mixed bag that contained advantages, such as the recognition of the postwar borders in Europe, as well as concessions, particularly on human rights.

The other significant results of that policy, aside from the dialogue with the United States in the nuclear field, were the agreements of the early 1970s with West Germany and the World War II allies on Berlin that effectively defused Europe's most explosive issue.

At home, the economy has started to decline after a period of growth and relative prosperity. Soviet technology, outside of military weaponry and possibly space, is woefully behind the West, Soviet agriculture has failed to feed the country, causing the government to become dependent on foreign grain suppliers.

Nevertheless, Brezhnev's tenure must be acknowledged as the best two decades the Soviet Union has enjoyed since its birth 65 years ago. Under his leadership the Soviet Union reached strategic parity with the West, pursued the world's most active space program and opened up to Western technology.

For all the failures in grain harvests and slower growth rates, the population of the Soviet Union today is in the main better housed, better dressed and better fed than ever before.

Russia under Brezhnev remained as it always had been: a rigidly authoritarian society where expression is tightly controlled and political or cultural dissenters risk severe punishment. Yet Brezhnev's Kremlin was not Joseph Stalin's. Neither he nor the aged leadership that he symbolized was disposed to take risks.

Soviet relations with the United States in the Carter era were subject to much mutual recrimination and frustration. Differences over human rights and third world policies overshadowed limited progress in arms control. In late 1979, U.S.-Soviet detente collapsed under the weight of Moscow's invasion of Afghanistan. SALT II failed to be ratified by the Senate, Carter imposed a grain embargo and the United States boycotted the 1980 Moscow Olympics.

With the election of Ronald Reagan, superpower ties reached new highs of rhetorical vituperation. While little was left of the structure of accords, relations still managed to deteriorate. Guerrilla insurgencies in Central America, martial law in Poland, a nuclear balance tilting toward Moscow -- all were portrayed by the Reagan administration as part of a Brezhnev-led Soviet grand design.

Brezhnev's relations with other Communist countries were dominated by two factors.

There was the continuing feud with China which heated to the point of border bloodshed in 1969 and endured past the death of Mao. And there were tensions within the Warsaw Pact, symbolized by the current suppression of Solidarity, the Polish free trade union, and the invasion of Czechoslovakia in 1968. Czechoslovakia was almost certainly Brezhnev's darkest hour.

The Sino-Soviet rift was inherited from his predecessor, Nikita Khrushchev, but Brezhnev proved incapable of ending it despite periodic overtures to the Chinese that continued until his death.

In Eastern Europe the impact of the Czechoslovak tragedy reverberated for years. It gave rise to what is known as the "Brezhnev Doctrine" -- one of the few such doctrinal pronouncements with which his name is likely to be identified. The doctrine, enunciated by Brezhnev in a speech a few weeks after the invasion, qualifies the sovereignty of other Communist states.

During the Brezhnev era the Soviet Union has been quite successful in identifying itself with the policies and objectives of Third World countries that hold a voting majority in the United Nations, although many of them have become leery of Soviet designs following the invasion of Afghanistan.