Boris N. Yeltsin and the rest of Russia's leadership on Saturday rejected Soviet President Mikhail S. Gorbachev's plea for emergency powers as "impermissible" and said they will take all steps necessary to protect their giant republic's sovereignty.

In a bold action that makes an institutional collision over Gorbachev's request inevitable, Yeltsin and the Russian legislature's Presidium served notice that they intend to embark in the coming days on a radical overhaul of Russia's economy, whatever the national legislature and Gorbachev decide for the country as a whole.Under the urging of Yeltsin, Russia's populist president, the republic's legislature has endorsed a 500-day program that would sell off or lease most government-owned economic assets and rapidly replace state planning with the supply-and-demand forces of a market.

Meanwhile, national lawmakers charged with choosing a reform plan for all 15 Soviet republics have dithered. Painting a dire picture of the country's "emergency," Gorbachev formally asked them Friday to grant him the power to issue decrees, in violation of existing law if necessary, on a broad range of topics ranging from economic management to the defense of law and order.

Gorbachev explicitly told the Supreme Soviet, as the national legislature is called, that he might have to institute direct "presidential rule" in certain places, shutting down, if necessary, local institutions, including elected bodies of government.

In the new Soviet political climate, where greater self-rule has become a loudly proclaimed goal and a right demanded from Kiev to Kazakhstan, those were fighting words, and with its statement read on nationwide television news, the top executive body in Russia's government served notice that if the Supreme Soviet grants Gorbachev's wish, it will not submit.

"The granting of the requested powers to the president of the USSR is impermissible," the Presidium of the Russian Republic's Supreme Soviet declared.

In the event Gorbachev is given such powers, the Russian government "will take all necessary measures to defend the sovereignty and the constitutional system of the R.S.F.S.R. (Russian Republic)," it said.

The statement, signed "B.N. Yeltsin," imperiled the uneasy but vastly important political alliance between the highly popular political maverickand the Soviet president.