KAZAN, U.S.S.R. -- When Czar Ivan the Terrible captured this Volga River city from the Tatars in 1552, he opened the way to more than four centuries of almost uninterrupted Russian expansionism. Today, the multinational empire founded by the czars and consolidated by the commissars is falling apart.

"This city is where the Russian empire started. It is also where it is going to end," predicted Fanil Ahmadi, a green-robed Muslim cleric, at a Tatar independence rally outside the parliament building here in Kazan, capital of the Russian autonomous republic of Tatarstan. "We were the first Muslim state to be incorporated into Russia. Once we get our freedom, all the others will demand their freedom as well."

The collapse of empire -- the historical process that got underway in earnest in 1989 with the toppling of pro-Soviet regimes in Eastern Europe and then spread to the Soviet Union itself -- has now reached the core of the centuries-old Russian state. The Russian federated republic, a vast patchwork of different nationalities, is being riven by much the same kind of ethnic disputes that led to the secession from the Soviet Union of the three Baltic states and determined independence movements in the Ukraine, Moldavia and Georgia.

Here in Tatarstan, one of 16 autonomous republics within the Russian federation, nationalists have stepped up their drive for full independence in the wake of August's abortive coup by hard-line Soviet Communists. Earlier this month, on the 439th anniversary of the fall of Kazan's white-walled kremlin, or citadel, to Russian troops, there were violent clashes outside the Tatar parliament as thousands of nationalists tried unsuccessfully to storm the building.

Independence fever has spread to a string of Muslim-inhabited autonomous republics in the northern Caucasus, a region that has been under Russian protection since the late 18th century. In the Chechen-Ingush republic, a retired air force general has mobilized a national guard of 60,000 people and called on them to prepare for war with Russia. Neighboring Dagestan also has been swept by nationalist disturbances.

"The big empire that was called the Soviet Union has already collapsed. Now it is the turn of the small empire called the RSFSR," said Rashab Safin, a Tatar leader, referring to the initials of the Russian Soviet Federated Socialist Republic. "What is happening is historically inevitable. Not a single empire survives forever. They all collapse -- and this one will collapse as well."

Consumed by their campaign for national independence, Tatar activists have so far paid relatively little attention to the vital question of how Russians will react to the loss of territory long regarded as an integral part of Mother Russia. But there already are signs that the final stage in the disintegration of the world's second superpower could turn out to be the most perilous and bitterly contested of all.

The resurgence of national feeling among Tatars and Ukrainians, Balts and Chechens, has coincided with the rebirth of Russia itself, the Soviet Union's most populous republic with 147 million people. Deliberately suppressed by the Communists for decades, Russian national pride was given a tremendous boost by the central role played by the Russian parliament and Russian President Boris Yeltsin in defeating the August takeover plot. Sooner or later, these rival national movements seem destined to clash.

Russian leaders who acquiesced in the defection of Eastern Europe and the Baltic states are likely to adopt a quite different position as the unraveling of empire comes closer to home. Shortly after the coup, Yeltsin hinted that Russia could have large territorial claims against the Ukraine and Kazakhstan if they pushed ahead with full independence. The rights of the 27 million or so Russians living in other republics have become a sensitive political issue within Russia itself.

The best-case scenario is that the old Russian empire eventually will be reorganized as a voluntary confederation of sovereign states, a kind of Eurasian version of the European Community, in which the rights of national minorities will be fully protected. The nightmare scenario is Yugoslavia writ large, a bloody civil war accompanied by the mutation of communism into nationalism.

"If the present wave of disintegration is not stopped, it will be the end of Russian history," warned emigre journalist Vladimir Maksimov in a recent interview with a Soviet newspaper. "It may be the end of human history as well" said Maksimov, who was stripped of his Soviet citizenship in 1976. "A cornered and hungry Russia with a huge nuclear arsenal could react to its global humiliation in a most unpredictable way." Ethnic Lines Are Blurred

To celebrate his capture of Kazan after a brutal five-year war, Ivan the Terrible ordered the construction of a splendid cathedral on Moscow's Red Square. With its twirling spires and fantastically colored bulbous domes, St. Basil's Cathedral has become a symbol of the cultural and ethnic diversity of a multinational state that straddles both Europe and Asia.

The Russian empire was never a conventional empire. In Russia, unlike the British or French empires, it is impossible to tell where the metropolis ends and the colonies begin. Colonizers and colonized have married and lived alongside each other for centuries. There are very few pure Russians, or pure Tatars, or pure Ukrainians.

Tatarstan is a case in point. The ethnic composition of its 1.7 million people is 47 percent Tatar and 43 percent Russian, with Chuvash and other ethnic groups accounting for the remaining 10 percent. Seven of every 10 Tatars -- more than 5 million people -- live outside their titular republic.

Aware that Tatarstan in its present borders is a largely artificial political entity, Tatar nationalists are already staking their claim to neighboring Russian regions, hoping to link up eventually with the Muslim republic of Kazakhstan. Much of this land is rich in oil and mineral resources, adding a significant economic dimension to the dispute with Moscow.

"These territories are ours. Russia is the golden circle around Moscow -- Vladimir, Tula, Yaroslavl," said Fauzia Bairamova, leader of the main nationalist faction in the Tatarstan parliament. "All this must be settled in peace negotiations. We are still in a state of war with Russia. Since 1552, there has been no peace treaty between Tatarstan and Russia."

The precarious position of Russians outside Russia proper has provided Russian nationalists with a ready-made political cause. In the Russian presidential election last spring, an extreme right-wing politician named Vladimir Zhirinovsky won 6 million votes by promising to create a unitary Russian state from the Baltic Sea to the Bering Strait. More moderate politicians, including the mayor of Moscow, Gavril Popov, have recently called for the "liquidation" of Tatarstan and other national-territorial units within Russia.

The potential for a Russian nationalist backlash has been further fueled by the spectacle of hundreds of thousands of Russians fleeing ethnic violence and job discrimination in the Transcaucasus, Central Asia and Moldavia. In Moldavia, ethnic Russians have formed their own breakaway "Dniester Soviet Socialist Republic" on the east bank of the Dniester River in defiance of both Moscow and Kishinev, the Moldavian capital.

"We Russians are like a big bear. Usually, we just stay silent. We're not easily provoked. But once we get aroused, it's very difficult to calm us down," said Anna Fedotar, one of several hundred Russian women who blocked railway lines leading into Moldavia for a month this fall to protest the arrest of Dniester leaders by Moldavian authorities.

Since he became Russia's first directly elected president in July, Yeltsin has used a mixture of carrot and stick in dealing with ethnic unrest. He has warned nationalists in Tatarstan and the Caucasus to disarm or face arrest, while promising that he will respect their desire for political autonomy.

Addressing the Russian parliament Monday, Yeltsin said he would not "allow the collapse of Russia and its disintegration into dozens of individual principalities feuding among themselves. We've already had such a period in our history, and it cost us very dearly." So far, however, his attempts to hold the country together show few signs of being more successful than those of Gorbachev.

"Both Yeltsin and Gorbachev have lost control here," said Khasbulat Shamsutdinov, editor of Vecherni Kazan, a Russian-language daily in Tatarstan. "Any attempt by Yeltsin to interfere in what is happening here will make things worse. If Yeltsin sent troops to Tatarstan, or even a threatening letter, it would strengthen the position of the Tatar nationalists."

When Yeltsin gave the leaders of Chechen-Ingush three days to disarm its national guard, they laughed. One Chechen activist, Khusain Akhmadov, described the Russian leader's threat as "the last belch of the Russian empire." The ultimatum expired last week with Moscow taking no significant action and the rebel republic of 1.3 million people electing a nationalist general, Dzakhar Dudayev, as president in a vote that Moscow described as illegal.

But Yeltsin has not yet shown his full hand, and some political commentators suspect that, if all else fails, he could be tempted to play the card of aggrieved Russian nationalism. Such a development would dismay his liberal supporters. But it would go down well with the still influential military-industrial complex and might deflect public attention from the growing economic crisis.

"Yeltsin is the product of the one-party state. He is an imperialist to his bones. The democratic gloss is very thin," gloated Alexander Prokhanov, a nationalist Russian writer and one of the ideologists of the failed August coup. "We are already seeing the beginning of Yeltsin's conversion from a democrat to a national leader. He no longer speaks about sovereignty for nations inside Russia. If he acquires absolute power, he could become another Stalin."

"There are signs that Yeltsin could be influenced in this direction. He is receiving different kinds of advice," said Sergei Kovalev, a liberal Russian legislator and friend of the late human-rights campaigner Andrei Sakharov. "I hope and trust that Yeltsin will withstand the pressure. If Russian nationalism were to rise like a wave and sweep over us, it would be another huge tragedy. It would lead to enormous bloodshed."

Time is running out for Yeltsin to leave a decisive stamp on Mother Russia. Unless he succeeds in defusing the nationality time bomb and putting the country firmly on the road toward a market economy in the next two years, he could be swept away by the primitive populist and nationalist forces that he has sought to manipulate.

There is a growing sense here that if democracy fails to take hold in Russia the alternative will not be communism but fascism. The coup probably dealt a death blow to Marxist ideology and the 16-million-member Communist Party. But nationalism remains a potent ideological force whose appeal could grow as the economic recession deepens.

"Nationalistic trends, Nazi trends are growing stronger and stronger," warned Anatoly Sobchak, the mayor of St. Petersburg, in a recent interview with a Soviet newspaper. "If democratic governments in the republics, the local authorities and the leadership in Moscow cannot stop inflation and falling living standards, then disenchantment and disaffection with democracy will set in."

Nearly three months after the abortive coup, Yeltsin finds himself in a serious political bind. If he pushes ahead with serious economic reform, freeing prices and closing money-losing factories, he could undermine his own popularity and trigger a possibly uncontrollable wave of social discontent. But if he fails to make good on his latest promises, the economic crisis will deepen to the point at which a dictatorship dressed up in national colors may prove the only way out. 'The Same Path as Yugoslavia?'

A recent confidential memorandum prepared by the KGB security police and leaked to the Soviet press warned that ethnic clashes and extensive internal migration could lead to the establishment of "national regimes of a populist, semi-fascist type." It added that the Soviet Union is going down "the same path as Yugoslavia, repeating almost step by step events that happened there a year to 18 months ago."

There are some obvious parallels between Yugoslavia and the Soviet Union. Both are multinational states with largely artificial borders, held together for decades by a now discredited communist ideology. Serbia, the largest Yugoslav republic, has its own troublesome autonomous regions, rather like Russia. Millions of Serbs live in other republics, just as millions of Russians live in the Ukraine, Kazakhstan and the Baltic states. Many Serbs and Russians are convinced that it was the big nations that suffered most under communism.

"That was the Communist system," said Vuk Draskovic, leader of Serbia's National Revival Party. "Given a choice between rich and poor, the Communists will always side with the poor. If the choice is between clever and stupid, they will side with the stupid. Between a big nation with a rich culture and a small nation with a little culture, they will always choose the small nation."

"Russians have been insulted for 70 years," said Zhirinovsky, the leader of the Russian neo-fascist movement. "The Communists, from Lenin to Gorbachev, deliberately tried to destroy the Russian state."

But while the Soviet Union could go the same way as Yugoslavia, such an outcome is by no means inevitable. Much will depend on the statesmanship displayed by political leaders as they seek to rebuild the country on a new basis, while at the same time making the transition from a centrally planned economy to a market economy.

After 70 years of communism, many Russians are tired of an imperial burden that has left them worse off than many of the nations they conquered. There is little popular enthusiasm for a "Greater Russia," let alone foreign wars. Polls suggest that a significant majority of Russians probably would be prepared to accept a shrunken Russia, stripped of its superpower status, if they could be assured of a better life.

"If the Russians are sensible, they can avoid our tragedy," said Zoran Djingic, a leader of the opposition Serbian Democratic Party. "The Russian empire is older, and less artificial, than Yugoslavia. We Serbs have more political complexes than Russians; we are more sensitive to territory, physical boundaries. Until the last century, the Serbian state existed only in myth. Russia has existed for 1,000 years -- and will continue to exist long after communism has been forgotten."