Soviet President Nikolai Podgorny, one of the most powerful men in the Kremlin for more than a decade, was dropped from the Communist Party Politburo today, signaling the end of his career.

Podgorny, 74, was not immediately deprived of his post as president, the country's ceremonial head of state, but that will surely follow.

The action announced at the end of a day-long plenum of the party's Central Committee was unexpected and could mark the start of the most significant shakeup at the very top level of Soviet leadership since the ouster of Nikita Khrushchev in October, 1964.

In another high-level change, the plenum dropped Konstantin Katushev, 48, from the senior party leadership, thereby removing the person who for many years has been the youngest of the Kremlin's top officials. He was replaced as the party secretary in charge of relations with ruling Communist regimes by Konstantin Rusakov, 68, a personal aide to party chief Leonid Brezhnev.

The plenum also adopted a draft constitution for the Soviet Union, ending an internal ideological debate that has gone on for 15 years over how to replace the charter drawn up in 1936 by Joseph Stalin. According to the official Tass News Agency, Brezhnev said the new constitution reflects the attainment of "mature socialism" in the U.S.S.R. But he only hinted at its contents. The text is expected to be published in the next few days.

If there is one common thread to the day's events, it seems to be that Brezhnev - for all the recurring rumors of his failing health - has increased his already enormous dominance of Soviet affairs.

The firing of Podgorny - there is no indication that he left willingly although some explanation such as advanced age or ill health may be offered when he leaves the presidency - reduces the number of full Politburo members to 14.

The main significance, for outsiders, is how today's changes affect Brezhnev's position and the inevitable succession to the 70-year-old party chief.

Podgorny's exit from the Politburo could well give Brezhnev the chance to place a known backer in the presidency, when it becomes open. Another possibility is that Premier Alexei Kosygin, who had a heart attack last summer and is 73, will be named to the presidency leaving open the potentially even more important title of premier. Finally, Brezhnev could take on the job himself so that he would have a state as well as party stature as Khrushchev did.

Katushev, who continues in the far lesser post of a deputy prime minister with responsibility for comecon, the Soviet bloc's economic alliance, was regarded as a Brezhnev protege - one of the very few figures in the Kremlin gerontocracy who could be considered a top leader in the next generation.

Katushev's failure was thought to be that he mishandled the preparation for last summer's conference in Berlin of European Communist parties. There the important Western Communist parties successfully challenged Soviet ideological primacy in the international movement. Soviet policy before that meeting went through so many flip-flops that the occasion turned out to be much more of an embarrassment to the Kremlin that it need have been.

Rusakov, the new secretary for relations with foreign ruling parties, is known to be an old Brezhnev crony, one of what is called the "Dnieper Mafia" after the region of the Ukraine where the party leader got his political start. Increasingly, Brezhnev is placing his friends from the Ukraine into positions of formidable influence.

Although it is the least easily grasped, perhaps the most important evidence of Brezhnev's still ascending authority to emerge today is the presentation of the constitution - a document that is meant, like the American constitution to embody the governing principles of the state and the basic rights of its citizens.

Work on the document began in 1962 and was continued by Brezhnev. On a number of occasions beginning in 1966 he predicted completion dates that proved wrong. Western analysts speculated that the delays meant that the party leader's ideas for so basic a doctrinal pronouncement were meeting resistence.

Exactly what the differences were is really only a matter of conjecture since the shadings of rhetoric - to drop all reference to the "dictatorship of the proletariat" in favor of "state of the entire people" as apparently has been done, for instance - mean little to outsiders.

The old constitution, written for Stalin just as he began the sweeping purges and repressions of the late 1930s, contained such enlightened principles as freedom of press, assembly and religion as well as the right of any of the 15 constituent Soviet republics such as the Ukraine, Armenia or Georgia to secede. How these matters are handled in the version will be of interest.

Brezhnev told the plenum that the constitution would reflect the "further expansion and deepening of socialist democracy" and, he said, there is "extensive elaboration . . . of provisions concerning the right of Soviet citizens including their socio-economic rights, civil rights and freedoms along with duties of citizens to the state."

The constitution will also reflect, said Brezhnev, the great changes in the world since the old one was drafted, as viewed from the Kremlin:

"The capitalist encirclement of the U.S.S.R. exists no more. Socialism has turned into a world system and . . . the positions of world capitalism have been substantially weakened. Dozen of young states, former colonies, are coming out against imperialism. A real possibility has appeared of preventing a new world war . . ."

Presumably that kind of commentary has been translated into theoretical notions about the structure of Soviet society.

After the document is published, it will be submitted to national "debate" before ratification. Description of it as a draft adopted "in the main" by the plenum indicate that there still may be slight modifications.

At one stage in 1973, Brezhnev indicated in a major address that the constitution would go to a national referendum. That mention was omitted from the speech when it was published. Kremlinologists said this showed that some Politburo members opposed the notion. There is no reference to a referendum in the plenum report.