TALLINN, U.S.S.R.— A PARADOX comes to life here in Estonia, an example of a life and death question for the Soviet Union. Mikhail S. Gorbachev has taken off many restraints that have held the country back, but have also held it together. Will it fly apart without them?

Even more than the other Baltic republics, here in the region that lies between Russia and Moscow's East European allies, Estonia is taking actions whose logic seems to be leading to a gradual secession from the Soviet Union. But logic in the Soviet Union has its own rules. Since the Constitution says that any of the 15 republics can secede any time, it must be impossible.

Nevertheless, people here have become unafraid to raise the possibility of independence. And some of them openly demand it. An Estonian independence party has declared itself, for example. So far, it has not been banned and its leaders have not been jailed.

''I don't know a single youth group that doesn't believe independence - secession from the Soviet Union -is the only way to preserve the continued national existence of Estonia,'' said Rumbold Mardo, the 21-year-old firebrand editor of an underground journal.

Indeed, the lid is off the nearly 50 years of resentment in Estonia, Latvia and Lithuania at the way Stalin seized them and incorporated them into the Soviet Union in 1940. Eastern Europeans might well sympathize.

The basic gamble of the Gorbachev policies is that more freedom will eventually lead to economic success, which could be an example for the rest of the Soviet economy. But the new latitude also brought out Baltic national pride that had long been suppressed, and the local authorities barely managed to catch the tide in time.

In Estonia, new party leaders replaced discredited ones and gave their blessing to the broad-based national political movement. The Estonian Legislature enacted a law requiring that Estonian be the language for all party, Government and business meetings, except where the overwhelming majority of people are not Estonian speakers. The last census figures, from 1979, say there were 947,812 Estonians and 408,778 ethnic Russians in the republic, and 108,000 people of other nationalities.

Russians and Estonians both say they view the language law as largely symbolic. Nobody really expects all Russians in public service to learn Estonian in the next four years, as the law says they must do.

But for some Russians here, the world seems to have turned upside down. They have formed their own organization, the Inter-Movement, whose leaders can scarcely conceal their bewilderment and dismay at being brusquely thrown out of the driver's seat - and at the lack of support they have been getting from Moscow. ''Maybe they're trying an experiment, to see how well we can work out our own problems in Estonia and carry out economic restructuring,'' said Viktor Y. Kiemets, a Russian engineer who said he joined the group last summer. ''But it's a peculiar feeling to be used as guinea pigs.'' Another Inter-Movement leader, Yuri Y. Rudyak, was more combative. ''We admit that Stalin was a criminal,'' he said. ''But we think we should remind the Estonian people that they first got real independence through Soviet power.'' Not many Estonians agree with him, if the letters Estonian journalists say are frequently published in the papers in Tallinn are any guide. They recall the brief period of independence from 1920 to 1940, formerly excoriated as the days of ''the bourgeois republic,'' as halcyon days of peace and prosperity.

And bit by bit, the Estonian Government itself seems to be cutting itself off from the Soviet Union.

Since the beginning of the year, a decree of the Estonian Government has ''temporarily'' barred private citizens and business cooperatives from sending scarce consumer items to friends, relatives and customers elsewhere in the Soviet Union, except to soldiers and sailors and earthquake victims in Armenia. The ban does not apply to official contracts to supply the Soviet state. But the fact that such a law exists at all is a blow to the concept of one internal Soviet market.

Piter Polluveer, a musician in the Gothic Lutheran Cathedral Church of St. Mary, which overlooks the old city here, summarized the situation here this way: ''Unless the Soviet Union falls apart altogether, there will be no independence for Estonians. But we are a stubborn nation. We have stayed on this territory for 5,000 years, and we've always waited out the bad times.''

Photo of demonstration for independence in Tallinn, Estonia (Visions/Kalju Suur)