MOSCOW, AUG. 1 -- Soviet President Mikhail Gorbachev has reached agreement in principle with his chief political rival, populist Russian leader Boris Yeltsin, on guidelines for a new economic reform package to be introduced this fall.

In an interview today, Gennadi Filshin, Deputy Premier of the Soviet Union's giant Russian republic, said that the new plan is based largely on a proposal drawn up last month by Yeltsin's aides for a 500-day transition from a centrally planned economy to a market system. But Filshin indicated that controversial aspects of the plan calling for removal of price controls would be implemented cautiously to guard against the possibility of an explosive social reaction.

Gorbachev's top economic adviser, Nikolai Petrakov, described the move by the two leaders to combine their efforts toward solving the nation's staggering economic problems "the most important information of 1990" in Soviet domestic politics. He said Gorbachev and Yeltsin signed the accord Saturday after several days of talks.

The tentative agreement comes amid harsh, widespread criticism of the Soviet government's own economic reform proposals and marks a significant political shift for Gorbachev, who campaigned strongly against Yeltsin's election as president of the Russian republic in June, accusing his former protege of demagoguery and of seeking to break up the Soviet Union.

Mikhail Poltoranin, information minister of the Russian republic and a close Yeltsin ally, said today that there had been a "narrowing of differences" between Yeltsin and Gorbachev over the past few weeks. As a politician with a large and loyal popular following, Yeltsin's support could be a valuable asset to the Soviet leader in a period of mounting social discontent and deepening distrust of the government.

Last week, during a meeting with radical-reformist intellectuals, Gorbachev called for an alliance of "center-left" forces to defend his reform movement against newly resurgent conservative forces in the ruling Communist Party. He also indicated then, without going into detail, that agreement had been reached between Kremlin moderates and Russian government officials on the outlines of a new economic reform program.

Filshin, who is responsible for supervising the Russian economy, said the new reform package will be "much more radical" than the one introduced by Soviet Premier Nikolai Ryzhkov this spring and would include a sweeping denationalization program designed to force competititon into the economy.

The Russian economic outline calls for the sale of 200 billion rubles worth of state assets and housing, closure or merger of a number of state ministries and the partial decollectivization of agriculture. It also envisions abolition of government subsidies on most commodities and the likelihood of a slump in industrial output as unprofitable enterprises are forced out of business.

Filshin suggested, however, that a removal of price controls is likely to be postponed to a later stage of the program. "You can imagine the social and political and economic consequences if we abruptly announced a transition to the market at a time of such terrible shortages," he said.

The Soviet Interfax news agency reported tonight that a group of economic specialists had been formed to work out procedures to bring a market system into operation by September. The group will include Gorbachev's chief economic advisers, Petrakov and Stanislav Shatalin, in addition to Grigori Yavlinsky, author of the original Russian program.

The Soviet government plan included proposals for doubling and tripling the prices of bread and other staple commodities, along with generous compensation arrangements to offset the impact of the price increases. The plan was met with scathing criticism around the country and was effectively shelved after a wave of panic buying broke out.

Legislators in the Russian republic, meanwhile, are pressing ahead with plans to legalize private ownership of land and dismantle collective farms. In some regions of the country, including the Yaroslavl area northeast of Moscow, laws have already been adopted that will oblige collective farm managers to give land to peasants who want to start their own farms.

In another development reflecting the gradual shift of power away from Moscow, the Interior Ministry agreed today to allow Soviet Estonia to take full responsibility for the maintenance of law and order within its own borders and that Soviet internal security troops could be sent there only at the request of the Estonian government.