MOSCOW, SEPT. 11 -- President Mikhail Gorbachev told the Supreme Soviet in a dramatic session today that he endorsed a radical plan of economic reform and decentralization, a direct rebuke of his prime minister, Nikolai Ryzhkov.

Gorbachev said that some of Ryzhkov's proposals, which rely heavily on traditional systems of centralized planning and state-owned property, could be incorporated into a "unified" program. But the author of the radical "500 Days" program, economist Stanislav Shatalin, said in an interview that the "unified" program is "99 percent mine" and includes only "five or six figures" from Ryzhkov's version.

If enacted by the Supreme Soviet and the legislatures of the constituent republics, the "500 Days" program would mark a historic break with a legacy of collectivist, centralized Stalinist economics.

The plan calls for a massive sell-off of state properties, decentralization of economic power from Moscow to the republics, the rise of private property and the creation of a stock market and other institutions found in Western market economies.

The legislature of the huge Russian republic endorsed the outline of the Shatalin program today with only one deputy voting against it. Other republics are expected to follow Russia's lead. Discussions in the Supreme Soviet on the "unified" plan, as well as the original Ryzhkov and Shatalin versions, are expected to begin Wednesday.

In an emotional speech, Gorbachev defended Ryzhkov against personal attacks and demands that the prime minister step down. "This smells very bad," Gorbachev said at one point. "This is not what the political process is about." But Gorbachev's public endorsement of Shatalin was unmistakable, and Ryzhkov indicated at a press conference later that he would resign his post if the final plan was too distant from his own.

"I can only fulfill my responsibilities if I believe in what I am doing," Ryzhkov said. "If I don't believe in {the program}, then I won't go near it." The Tass news agency, a normally staid organ of the state, summarized Ryzhkov's message as "Back Me or Sack Me."

Ryzhkov said that he favors a gradual transfer to a market system and that, in his view, Shatalin's proposal to transfer economic control to the republics "endangers the unity of the country."

"To be, or not to be, a united government, that is the question," Ryzhkov said.

He said that while his plan called for fixed price rises, Shatalin's idea of freeing many prices to market levels threatens to bring about far higher levels of inflation and social unrest.

"If market forces are just given free rein, then there will be a sharp decline in living standards," Ryzhkov said.

Asked whether he thought Ryzhkov should resign or try to enact a program he does not endorse, Shatalin said, "That's his problem."

Shatalin and other analysts agree that radical economic reform is bound to cause confusion among people used to years of the old system, along with a burst of inflation and "structural" unemployment when unprofitable plants are closed. Outside the Supreme Soviet building, Gorbachev told legislators and reporters, "No matter what program for a market economy we choose, we are in for hard times."

The legislators, or deputies, of the Supreme Soviet arrived today at the Kremlin with a sense of anticipation. On Monday they had been told that they would hear Ryzhkov present the "unified" program. Instead, Ryzhkov described an abbreviated version of the plan he had worked out with his aides and expressed doubts about a more radical transfer to the market. His speech said nothing about a "unified" plan.

When Ryzhkov finished, Supreme Soviet Chairman Anatoly Lukyanov asked if there were any questions. The deputies were stunned. Anatoly Sobchak, the mayor of Leningrad, was first at the microphone. "I've been listening and I don't understand what it is I've heard," he said. "Was that the {Ryzhkov} plan or the criticism of a proposal we have not even heard yet?"

Genrikh Igitian of Armenia mocked the competence of the Supreme Soviet and the government and Gorbachev as "that great master of improvisation." There was squabbling over what documents were available and repeated calls for Ryzhkov's resignation.

Even by the standards of the Supreme Soviet, which can be as rowdy as the British Parliament but has none of the experience or panache, the session was getting out of hand.

Gorbachev, who had been sitting off to the side of the presidium dais, finally stepped in. Using all his familiar gestures, theatrical pauses and alternations of high emotion and great gravity, Gorbachev endorsed Shatalin's policy while trying to protect Ryzhkov politically.

"I have the sense that Gorbachev really thinks that if Ryzhkov goes his government will be even more unstable," said Arkady Murashev, a leader of the Inter-Regional Group of radical deputies.

In recent months, Ryzhkov has come under attack in the press and in political circles. Today the youth newspaper Komsomolskaya Pravda accused him of "betraying" his anti-private-property principles by buying a country home for 47,000 rubles. Ryzhkov passionately defended his purchase, saying he had made it to avoid criticism for using a government dacha.

Gorbachev's old rival in the Communist Party, Yegor Ligachev, watched from the gallery and said in an interview later that he enjoyed the "back-and-forth on a theatrical level."

"It's better than Shakespeare!" Ligachev said.

The key political reason for Gorbachev's movement toward a more radical view of the economy has been the rising power of the Soviet republics and their demands for economic sovereignty. After years of intense political rivalry, Russian republic President Boris Yeltsin cooperated with Gorbachev in forming the Shatalin commission on economic reform in August.

The "500 Days" document outlines a strict timetable -- "one which we violate at our peril" -- Yeltsin has said under which economic reforms would be implemented in stages.