Reformer said to want `out' of the Kremlin. There are signs Boris Yeltsin is frustrated with opposition to reform and wants to resign from the Politburo. His departure would be a blow to the Soviet leadership's image of solidarity.

Reports are circulating here that Boris Yeltsin, the Soviet leadership's most outspoken supporter of political reform, has expressed a desire to resign from the Politburo, and has attacked Yegor Ligachev, the second-ranking Soviet leader, over his lack of support for reform. There is so far no indication that Mr. Yeltsin, who is often described as blunt and somewhat emotional, has actually submitted his resignation. Such an action would probably be unprecedented in modern Soviet history, and would be a serious blow to the leadership's carefully cultivated image of solidarity.

According to a well-informed Soviet source with regular access to the senior echelons of Soviet leadership, Yeltsin told an unscheduled plenary meeting of the Communist Party Central Committee Oct. 21 that he wanted to leave the Politburo.

Yeltsin claimed that perestroika (restructuring) had reached a dead end, the source said, and that Mr. Ligachev was slowing the process of social, political, and economic change.

Other speakers reportedly urged patience, and tried to discourage Yeltsin from following through with his threat. Contacted yesterday by the Monitor, an official of the Moscow city Communist Party said he had no information on the subject, but suggested we wait for an ``announcement.'' A party Central Committee staff member said categorically that he had not heard any rumors or reports about Yeltsin's speech.

The one-day plenum was officially described as routine. There are, however, several signs that it was not.

Recent Gorbachev-era plenums have been preceded by widespread debate of the key issues to be discussed by the Central Committee. No such discussion preceded this plenum. Recent plenums have been followed by the publication of resolutions adopted at the meetings, or by Soviet leader Mikhail Gorbachev's address to the gatherings. Nothing has been published from this meeting. Recent plenums have lasted two days; this meeting was finished in one.

The well-informed Soviet source says three issues were discussed at the meeting: Gorbachev's report to be presented at the 70th anniversary of the Bolshevik Revolution, the resignation of deputy prime minister Geidar Aliyev, and Yeltsin's request. Gorbachev's report was quickly endorsed, the source says, as was Mr. Aliyev's resignation. Yeltsin's announcement was the main subject of discussion.

The brief official announcement of the plenum mentioned only Gorbachev's speech and Aliyev's resignation. The list of speakers included in the announcement indicates that Yeltsin was first to speak after Gorbachev, and was followed by Ligachev. The list also shows that, of the 25 other speakers, three were Central Committee members affiliated either with the Moscow city or region administration.

At least five others were party officials thought to have close links with either Ligachev or Yeltsin.

The latter group included Prime Minister Nikolai Ryzhkov, Kazakhstan First Secretary Gennady Kolbin, and ambassador to France Yakov Ryabov, all of whom graduated, like Yeltsin, from the Ural Polytechnic Institute.

Gennady Bogomyakov, another speaker, held senior academic positions in the Siberian city of Novosibirsk while Ligachev was party first secretary of the region.

Yeltsin, born in January 1931, is just two months older than Gorbachev, and until 1985 spent most of his career in the city of Sverdlovsk. In April 1985, after Gorbachev's appointment as party leader, Yeltsin moved to Moscow. In December 1985, he replaced Viktor Grishin, reportedly one of Gorbachev's main rivals, as Moscow party chief. In February 1986 he became a candidate (nonvoting) member of the Politburo.

Since that time he has on several occasions been passed over for promotion to full membership of the Politburo. By contrast, Alexander Yakovlev, another of the prime movers behind the present reforms, was promoted to candidate and then full Politburo status in the space of one year.

Even Yeltsin's admirers, however, describe him as abrasive and ``extreme'' in his views. He has the reputation of a tough disciplinarian, and he has been quoted as saying that he works 20 hours a day.

Since the 27th party congress in 1986, there has also been speculation, mostly from non-Soviet sources, that Yeltsin and Ligachev are at odds. Yeltsin is depicted as desiring fast, thoroughgoing change, while Ligachev is seen as wanting more gradual, less ambitious reforms.