MOSCOW, OCT. 24 -- The legislatures of Russia and the Ukraine headed toward a constitutional showdown with the Kremlin today by insisting that their approval is needed before Soviet legislation can take effect in their territory.

This put the two largest republics directly at odds with the national legislature, or Supreme Soviet, in Moscow, which passed a resolution earlier in the day asserting that its laws take precedence. The Supreme Soviet formally struck down declarations of sovereignty by most of the 15 republics, which assert the supremacy of their own legislation.

The constitutional dispute between the republics and the central government is likely to complicate implementation of a new package of economic changes adopted by the Supreme Soviet last week after long hesitation. It could also jeopardize the effectiveness of decrees being planned by President Mikhail Gorbachev to stabilize the Soviet economy and slash the budget deficit.

Addressing a labor union congress today in Moscow, Gorbachev described the collapse of the federal structure of government as "the gravest danger" facing the Soviet Union. He added that there could be a return to a "strong-hand government" -- a euphemism for authoritarian rule -- unless the country succeeded in moving toward a market economy.

Areas of conflict between the central government and the republics include powers to tax and to control the exploitation of natural resources. Economic guidelines adopted by the Supreme Soviet last week give the central authorities almost exclusive control over export revenues, a point that is likely to be bitterly contested by most of the republics.

The resolution adopted today by the Russian legislature, or Soviet, covers all federal legislation as well as presidential "decrees and other decisions." The only exception is legislation in such areas as transport and defense, responsibility for which has been been specifically delegated to the central government by Russia.

In a speech last week, Russian President Boris Yeltsin accused Gorbachev of reneging on an agreement to implement a radical plan to create the foundations of a market economy in 500 days. He said it would be necessary to find ways of protecting the population of Russia -- half of the total Soviet population -- "from the negative effects of decisions taken by the center."

Constitutional amendments adopted by the Ukrainian Soviet include the legalization of a multi-party system and the dropping of clauses that ensure a special role for the Communist Party. Last week, the Ukrainian authorities were obliged to agree to hold a republic-wide referendum on whether the Communist-dominated legislature still enjoys popular confidence.

In another challenge to Moscow, the Estonian Soviet said today it would ignore a presidential decree ordering the holding of military parades on Nov. 7, the anniversary of the 1917 Bolshevik revolution. The non-Communist governments in neighboring Latvia and Lithuania are also attempting to ban parades, despite the opposition of the Soviet military. The Supreme Soviet in Moscow voted Tuesday to back Revolution Day festivities in the capital after city authorities refused to fund the parade. Other local bodies in many parts of the Ukraine and the southern Transcaucasus region also are boycotting the celebrations on both political and economic grounds.

In the southwestern republic of Moldavia, meanwhile, tensions between the ethnic Romanian majority and a Turkish minority known as the Gagauz are reaching a new pitch. Moldavian President Mircea Snegur appealed to the 300,000 Gagauz to call off elections planned for this weekend on the creation of an independent assembly, saying that the move could lead to "civil war."