Boris Yeltsin's backers will cripple Moscow with a one-day general strike if he fails to gain a parliamentary seat in the Soviet Union's first multi-candidate elections, a campaign organizer for the former local party boss warned Saturday.

The warning on the eve of historic voting for a new national legislature was issued as thousands of people rallied for Yeltsin and the maverick populist candidate accused the Communist Party of using undemocratic methods to block his bid for a seat in the congress.Voting in remote Siberia began Friday.

The voting Sunday, the first since the 1917 Bolshevik Revolution to give Soviet citizens a choice of candidates, is a centerpiece of Soviet leader Mikhail Gorbachev's bold program to democratize his country.

On the last day of campaigning for the elections, more than 10,000 people chanting "Yeltsin, Yeltsin!" attended his final rally in a parking lot outside the Lenin sports stadium.

Hundreds of police, some of them on horseback, watched the demonstrators but did not intervene in the largest gathering of Yeltsin's campaign, which was staged only 24 hours after its organizers received permission to hold it.

Soviet poet Yevgeny Yevtushenko addressed the rally and later told reporters that Yeltsin's campaign presented an unprecedented opportunity for democratization.

"Most people here today are working people, and this is the first time we have a choice, even if it is a limited choice," Yevtushenko said. "What we are witnessing here is history. We must make this start and move on to full free elections."

Yeltsin was fired from his posts as a Politburo member and as Moscow party boss in October 1987 after he criticized opponents of the pace of Gorbachev's ambitious reforms.

In his attempt to make a political comeback, Yeltsin is pitted Sunday against Yevgeny Brakov, a party-approved candidate and head of a major car factory in Moscow.

Yeltsin's populist platform has included fierce attacks on the privileges of the Communist Party elite. He has also urged Soviet leaders to consider permitting the formation of an opposition political party, a radical proposal that Gorbachev has not embraced.

In a move that Yeltsin's supporters have decried as part of a smear campaign against him, the party's Central Committee launched an investigation into vague allegations of unethical campaign practices.

Moscovskaya Pravda, the official newspaper of the Moscow branch of the Communist Party, previously published letters aimed at discrediting the former local party chief.

Many of his supporters, pointing to Yeltsin's challenge to the status quo and the controversy his campaign has stirred, have expressed fears that election officials may try to stuff the ballot box for Brakov or otherwise manipulate the final tally.

A total of 1,500 posts in the new 2,250-seat Congress of People's Deputies were to be contested Sunday.

A group of American lawyers and scholars sponsored by the Washington-based International Human Rights Law Group arrived in the Soviet Union to monitor voting in the Moscow area.

Some participants in Saturday's rally shouted out calls for the introduction of a multiparty system in the Soviet Union, and others responded with rousing choruses of "Yes, yes, we support it!"

One banner, expressing what has become the standard campaign reference to a multi-party system, read: "We are for socialist pluralism."

The Soviet Constitution bans any party other than the Communist organization. Yeltsin has been careful to stop short of actually supporting the establishment of an opposition party, saying only that Soviet leaders and citizens should debate the issue.

Yeltsin, apparently fearing that his appearance at the rally might spark confrontations between his supporters and police, did not speak to the crowd.