Alexei Shibayev, the Soviet Union's top trade union official and long considered a protege of President Leonid Brezhnev, was unexpectedly dismissed from his post today amid rumors of a power struggle in the Kremlin.

Soviet officials gave no explanation for Shibayev's ouster. The decision was made at a meeting of the trade union's central council and was announced by the government news agency Tass. Soviet sources said Shibayev, 67, was not present at the meeting.

Diplomats here speculated that the ouster reflects the current jockeying for position among Kremlin leaders following the death of Mikhail Suslov, the second-ranking official in the Soviet Communist Party. The ultimate prize is Brezhnev's post, once the 75-year-old Soviet leader steps down or is forced out.

There have been no clear indications here of any open challenge to Brezhnev. Western diplomats believe the Soviet president is likely to retain power as long as he remains in good health.

But speculation about Brezhnev's future has been fueled recently by a series of embarrassing rumors of government corruption involving members of the president's family, along with a published fictional article taken to be a scathing satire of the Soviet leader's public pronouncements and his continuance in office despite his advanced age.

Much of the speculation in informed circles centers on the sudden rise of Politburo member Konstantin Chernyenko, considered another Brezhnev protege, and the equally sudden fading from public view of Andrei Kirilenko, one of the party's senior figures.

The Associated Press reported Friday that Chernyenko attended the trade union council meeting at which Shibayev was ousted but gave no indication of his role in the decision.

Chernyenko, who became a member of the ruling Politburo less than four years ago, was prominently ranked at Suslov's funeral in January. By contrast, Kirilenko, a Politburo member for more than 20 years and long viewed as a possible successor to Brezhnev, was less prominent at the funeral and was not among the party of senior officials who greeted Polish leader Wojciech Jaruzelski this week.

Specialists here also noted that Kirilenko was the only Politburo member in Moscow to have been absent at a performance of a play about Lenin attended by Brezhnev and the rest of the Soviet leadership this week. Kirilenko was also missing from a Bolshoi Theater ceremony today honoring March 8 as Women's Day.

Union leader Shibayev's dismissal, which occurred only 10 days before the 17th Soviet Trade Union Congress begins meeting here, was seen as highly unusual. Only three days ago, the official Central Committee journal Agitator carried a long article by Shibayev about union plans.

Some Western observers speculated that, besides its political implications, the ouster also may be linked to Soviet efforts to slightly modify their trade union organization in the wake of the Polish crisis. The trade unions, which count 127 million Soviets as members, work closely with the party and the government.

Western analysts had identified Shibayev as a longtime ally of Brezhnev. As a Communist Party regional official from the Saratov district, Shibayev backed Brezhnev in the successful effort to remove Nikita Khrushchev from power in 1964. Shibayev was appointed chairman of the Soviet trade unions in 1976.

Shibayev, who has been a member of the the Communist Party's Central Committee for more than 20 years, was replaced by Stepan Shalayev, minister of pulp, paper and wood processing industries.