MOSCOW, FEB. 23 -- Russia's conservative parliament, in a direct challenge to President Boris Yeltsin, voted overwhelmingly today to grant a full amnesty to leaders of the 1991 Soviet coup attempt and the violent uprising of October 1993 that threatened to topple the Russian government.

The State Duma, as the lower house of parliament is known, led by Communists and extreme nationalists, voted 252 to 67, with 28 abstentions, to end the prosecutions of some of Yeltsin's fiercest opponents. The vote came on the day before Yeltsin was scheduled to make a key speech to the parliament outlining his program and the future of reforms.

Until now the Duma, which has been in session just six weeks, had launched regular rhetorical attacks against the president but had shied away from the sort of direct confrontations that led Yeltsin to dissolve the former parliament last fall. Today's action was the first overt sign that the lawmakers are prepared to launch a full frontal assault on a president whose political and physical health have been in question lately.

It was also a sign that Yeltsin, who sacrificed the two top economic reformers in his government after the reformers' poor showing in December's parliamentary elections, has bought himself little political breathing space with the move.

The constitution gives the Duma exclusive power to grant amnesties, and the resolution passed today declared the amnesties effective immediately. But given Yeltsin's strong opposition to an amnesty, it seemed unlikely that the ringleaders of the 1991 coup attempt and the 1993 rebellion would walk out of prison right away. It seemed possible that Yeltsin would somehow seek to forestall the Duma's action or ignore it.

But the Tass news agency quoted the prosecutor general's office late tonight as saying that it regards the Duma's vote as binding and that moves to release the amnestied leaders "will apparently begin tomorrow {Thursday}."

There were immediate and dire warnings from Yeltsin's allies that today's vote was aimed at destabilizing Russia and seizing power from Yeltsin's government and could lead to yet another constitutional crisis.

The president's chief spokesman, Vyacheslav Kostikov, said the Duma had "failed to draw lessons from the bitter experience" of the old parliament. He added: "This is a dangerous path that runs counter to Russia's interests. The sole responsibility for the consequences of this decision rests with the State Duma, which initiated this action."

In tone and content, Kostikov's remarks were nearly identical to statements by Yeltsin and government officials last fall when they ordered an assault by tanks and special forces troops on the defiant Supreme Soviet after it refused to disband.

Liberals in Yeltsin's camp challenged the legitimacy of the Duma's action, which they said would abort current legal proceedings. "This problem cannot be resolved by a resolution of a legislative or political organ," said Vladimir Shumeiko, a Yeltsin ally who heads the upper chamber of the parliament, the Federation Council. "It is not up to the legislators to interfere in the legal process."

The new Russian constitution, passed in a referendum last December, gives the upper house no role in the matter.

Yegor Gaidar, Yeltsin's former prime minister and economic guru, said the president has no power to veto the Duma's amnesty vote, which he called an "extremely dangerous decision."

"I am absolutely convinced the people released from prison will start forming groups of militants in the coming months and bring them onto the streets of Moscow," Gaidar told journalists.

Opponents of the government unleashed a barrage of rhetoric against the president. The leader of the Russian Communist Party, Gennady Zyuganov, called on the president to resign. The speaker of the Duma, longtime Communist Ivan Rybkin, called for the formation of a new government of "national reconciliation."

The action today came as nearly 1,000 ardent opponents of the government defied an order by Moscow city authorities and marched down the capital's main street, carrying Soviet flags and chanting anti-Yeltsin slogans.

Across town, several hundred supporters of radical nationalist Vladimir Zhirinovsky gathered in a large auditorium to hear their leader proclaim his vision of a Great Russia and unity between the Russians and their Serb allies in Bosnia.

Also today, the Russian ruble fell by 2.6 percent to an all-time low against the dollar on fears that a new government's spending plan will cripple the economy and send inflation soaring. Bankers said Russia's Central Bank, which has already spent billions of dollars this year defending the ruble, is running out of money and can no longer prop up the anemic currency.

On Tuesday, Central Bank chief Viktor Gerashchenko said in a news conference in Vienna that the government had "huge, enormous, looming problems" in drafting a budget. He said Russia may be forced to sell some of its gold reserves to meet expenses.

Yeltsin was originally scheduled to present his long-awaited economic and reform plan to the parliament last week, but the speech was delayed until Thursday because the president was said to be suffering from a bad cold. Ever since the December legislative elections, in which Communists and ultranationalists led by Zhirinovsky scored stunning gains, Yeltsin's political health also has been uncertain. The vote today suggested that his opponents, sensing the president's weakness, have decided the time is ripe for attack.

The amnesty, if carried out, would free many of Yeltsin's most bitter and potentially dangerous political enemies -- those who have demonstrated willingness to use extralegal means, including violence, to challenge his authority.

They include former vice president Alexander Rutskoi and parliament speaker Ruslan Khasbulatov, who set off last October's bloody uprising by refusing to disband the parliament in accordance with Yeltsin's order. They also urged their supporters to attack the main state television broadcast center, which led to a shootout that cost scores of lives.

The amnesty would also cover 12 high-ranking Communists who led the botched coup attempt in August 1991 against then-President Mikhail Gorbachev. The 12 plotters are in the midst of a 10-month-old trial on charges of trying to topple the government.