MOSCOW -- Radical members of the Soviet Union's new national assembly, organizing the first formal opposition group within the country's political system in nearly 70 years, Saturday called on President Mikhail S. Gorbachev to undertake even bolder reforms and lead the nation out of what they called a deepening crisis.

The radicals, all members of the Congress of People's Deputies, argued that the reforms undertaken up to now have been largely half-measures and consequently have not solved the country's structural political, economic and social problems--and in some cases have made them worse.

If perestroika , as Gorbachev's reforms are known, is to succeed, much more sweeping changes will be needed, the deputies said, starting with the replacement of unelected Communist Party and government officials who continue to wield great power but without direct responsibility to the people.

Loyal Opposition

The radicals, moreover, are offering, in a move that would substantially alter the top-down command structure of Soviet socialism, to serve as a critical but loyal opposition to the government in order to ensure the progress of perestroika .

Yuri N. Afanasyev, director of the Moscow Historical Archive Institute, appealed to Gorbachev to move away from his centrist position, where he has balanced the demands of the radicals and conservatives, and to forge ahead with more fundamental changes in the Soviet system.

"Gorbachev is regarded, quite justifiably, as the man who launched our reforms," Afanasyev declared. "But the time has passed when he can successfully remain the leader of perestroika and the leader of the nomenklatura . He has to make a choice."

'A New Class'

The nomenklatura --those party and government officials appointed by the party's powerful Central Committee--now constitute "a new class" in the Soviet Union, the radical populist leader Boris N. Yeltsin told the meeting, implying the need in terms of Marxist-Leninist politics for a "class struggle" to remove them from power.

The widespread strikes by Soviet coal miners this month "showed the growing social tension in the country and the fact that the working class has lost all faith in those who are supposed to be its leaders," Yeltsin said, praising the strike committees as "embryos of real people's power."

"The party aristocracy has become a class in itself," he added. "The workers' movement is directed against that class, even if it does not yet recognize this fact."

And Gavriil Popov, a prominent economist who proposed creation of an independent group of deputies two months ago, said that the miners' strikes had demonstrated the need for an alternative political voice. The strikes were not only justified, he said, but stemmed directly from the failure of Gorbachev's reforms to change the harsh day-to-day life of Soviet workers.

The deputies plan to create a formal structure today for what they call an Inter-Regional Group, which the official Tass news agency characterized as "left radical" in its orientation, as a way to put pressure on the Soviet leadership and to push their ideas throughout the legislative process.

Although the radicals deny that they will constitute a rival to the ruling Communist Party, of which most are members, or an opposition in the Western sense, the new group intends to criticize the government publicly and to work in the Congress of People's Deputies and the Supreme Soviet, the standing legislature, for policy changes.

"We are avoiding the words 'opposition' and 'faction' because they are perceived here so negatively," Yuri Andreyev, a Moscow deputy, told journalists. "In reality, this is a group of opposition deputies who want to work in an organized way at the Congress and at the Supreme Soviet."

Profound Changes Reflected

The apparent willingness of the Communist Party leadership to tolerate the new group is another reflection of the profound changes under way within the Soviet political system, and the group's creation against the background of widespread dissatisfaction with the party's performance appears likely to have a far-reaching impact.

Political "factionalism," as virtually any group criticism of official policy or advocacy of any alternatives is termed, has not been tolerated within the Soviet Communist Party since its 10th Congress in March, 1921. The last rival party, the Socialist Revolutionaries, was suppressed in 1922.

Although V. I. Lenin, the founder of the Soviet state, had said that it was "permissible . . . for different groups to organize in blocs" and seek support for their positions, particularly before party conferences, he grew so impatient with the radical Workers Opposition in 1920 and 1921 that in a pamphlet on "The Party Crisis" he declared, "The time has come to put an end to the opposition, to put a lid on it--we have had enough of opposition now!"