MOSCOW, DEC. 19 -- Soviet President Mikhail Gorbachev, responding to growing pressure for a crackdown on separatists and black marketeers, today reiterated earlier warnings that he would impose direct rule from Moscow in selected regions in the event of turmoil.

Gorbachev's warning, in a speech to the Congress of People's Deputies, followed the release of an extraordinary appeal from 53 military, industrial, and cultural leaders calling for tough measures to preserve the Soviet Union's communist system and territorial integrity. The signatories of the letter included the chief of staff of the armed forces, the head of the navy, and the patriarch of the Russian Orthodox Church.

It is the first time that top military leaders have joined forces with conservative Russian writers and Communist Party hard-liners to demand a crackdown against "separatists" and other "anti-Socialist" forces. It is dramatic evidence of the political pressure that has been mounting on Gorbachev behind the scenes to move toward a more authoritarian regime.

Tacit support for the conservatives came from Prime Minister Nikolai Ryzkhov, making what many deputies assumed to be his farewell speech prior to a major government reshuffle. While voicing his personal backing for Gorbachev, Ryzkhov said that perestroika, or restructuring, as initially conceived had "failed" and he accused his political opponents of attempting to destroy the entire Soviet system.

Addressing the Congress this afternoon, Gorbachev cited the Baltic states, the southern Transcaucasus region, and the southwestern republic of Moldavia as major trouble spots. But his warning that he might deal with the situation by making full use of extraordinary powers voted to him earlier this year did not go substantially beyond a Sept. 21 statement that he made to the Supreme Soviet, the smaller, standing legislature, which meets year-round.

"I would like to stress that where the situation becomes especially tense and a serious threat to the security of the state and the life of people is created, it will be necessary to introduce a state of emergency or presidential rule," Gorbachev declared today.

By repeating his earlier threat to impose presidential rule on certain regions, Gorbachev appeared to be attempting to appease the conservatives and frighten rebellious republics into falling into line. At the same time, there appears little doubt that he is ready to use his considerable repressive powers if the political and economic situation continues to deteriorate.

Russian Republic President Boris Yeltsin, generally regarded as Gorbachev's main political rival, accused the Kremlin of attempting to "retain its previous position as the omnipotent master" of all 15 Soviet republics. He said Gorbachev was amassing greater constitutional power than any previous Soviet leader, including Joseph Stalin.

"The center is seeking to shape a constitutional form of unlimited authoritarian rule, which may ultimately lead to a situation where arbitrary rule could be constitutionally justified," the populist Russian leader declared.

Constitutional powers granted to Gorbachev in recent months include authority to run the economy virtually by decree and to suspend the legislatures of individual republics in the event of a threat to the security of Soviet citizens. KGB and Interior Ministry troops who would bear the brunt of enforcing presidential rule have been built up through recruiting drives and the transfer of crack divisions from the army.

In his 45-minute speech, carried live on state television, Gorbachev warned that 1991 was likely to be a year of "unpopular but necessary decisions" in the political and economic fields. In a clear attack on Yeltsin, he drew applause from among the more than 2,000 deputies when he condemned "rotten populism" that he said was attempting to set the republics against the center.

Criticizing nationalist leaders in the Baltic republics of Lithuania, Latvia, and Estonia, Gorbachev said he was worried by attempts to deprive non-Baltic people living there of their citizenship rights, which he called a violation of human rights. The assemblies of the three Baltic states have passed laws restricting citizenship rights to long-term residents.

Baltic deputies, who have been bracing for some kind of Kremlin crackdown for months, brushed aside Gorbachev's latest warnings. One Latvian legislator, Nikolai Neiland, said the president had qualified his threats with "many ifs" and had not said anything substantially new.

Baltic leaders already have discussed the possibility of civil disobedience campaigns if the Kremlin attempts to cut short their drive for independence. The deputy speaker of Estonia's assembly, Marju Lauristin, said she expected that "sooner or later" Gorbachev would declare presidential rule or martial law in one or more of the Baltic republics.

"We will not deliberately organize demonstrations or provoke the situation, but we will resist peacefully and not participate in any fake parliaments," she said.

The military leaders calling for a crackdown included armed forces chief of staff, Gen. Mikhail Moiseyev, ground forces chief Gen. Valentin Varennikov, and former Warsaw Pact commander Gen. Viktor Kulikov. They were joined by conservative Russian writers Yuri Bondarev, Vasily Belov and Alexander Prokhanov, and two hard-line Communist Party secretaries, Oleg Baklanov and Boris Gidaspov. Culture Minister Nikolai Gubenko, previously considered a liberal, also signed.

Warning that perestroika is getting "bogged down in a ruinous darkness," the signers said the Soviet Union risked "losing its social system that has cost the nation enormous suffering and sacrifice, but which has been preserved despite all the cataclysms for the last 70 years." It said the country was threatened by a "ruthless dictatorship" of separatists and black market dealers.