Calling the country's new constitution the "outcome of the entire 60 years development of the Soviet State," Soviet President and Communist Party leader Leonid Brezhnev opened a special four-day session of the national legislature today with a defense of the document as superior to any constitution in the world.

Brezhnev derided Western criticism of the new instrument.

"The critics of the Soviet constitution have found themselves in an unenviable position," he declared."They cannot escape the fact that the Soviet draft constitution defines the social, economic and political rights and freedoms of citizens and the specific guarantees of these rights more widely, clearly and fully than ever and anywhere else."

Whereas the new Soviet law guarantees a job, a home, a pension, a vacation, free medical care and other specific rihts, Brezhnev said sarcastically, the West offers such "rights" as unemployment, costly medical care, racial discrimination and organized crime. "propagandists and ideologists of capitalism cannot deny that socialism has long cured these social sores," he asserted.

The new document, first promulgated earlier this year, is meant, like the U.S. Constitution, to embody the governing principles of the state and the basic rights of its citizens. The law will be adopted by the Supreme Soviet, the bicarmeral legislature, on Friday, after three more days' discussion of the draft.

The law is divided into nine sections and 173 articles in its draft form, with articles ranging from such declaration as "all power in the U.S.S.R. shall be vested in the people," to a definition of the separate union and autonomous republics.

In his 90-minute prepared speech, Brezhnev said that 140 million men and women, 80 per cent of the nation's adult population, had discussed the new charter. "Never before has this country had popular activity on such a scale," he claimed.

The new law, unofficially called the "Brezhnev constitution," will replace the so-called "Stalin constitution" adopted in 1936. The new constitution guarantees that the Communist Party occupies the supreme position of importance to the state.

It includes a new position, that of first deputy chairman of the presidium, or vice president, which Brezhnev did not mention today but which has been the center of considerable speculation among foreigners here over who will fill it.

Some sources have suggested that the occupant could be in line to succeed the aging Brezhnev. Yesterday, the Communist Party Centeral Committee promoted a Brezhnev confidant, Konstantin Chernyenko, and a longtime diplomat, Vasili Kuznetsov, to new posts as candidate (nonvoting) members of the powerful Politburo. Sources have suggested that Kuznetsov may be elevated to the new post.

A polished diplomat, Kuznetsov could relieve Brezhnev of the heavy protocol demands of the presidency. At 76, however, Kuznetsov would not figure in any succession role. Others who have been mentioned for the post include Vladimir Scherbitsky, Ukrainian party chief; Foreign Minister Andrei Gromyko; and Chernyenko, a party administrator.But all this is speculation among foreigners who have almost no contact with the inner leadership.

Specific changes in the draft that Brezhnev said will be considered by the Supreme Soviet include a strong article condemning and calling for punishment of those who shirk work; requiring children to care for parents as well as the other way around; and providing that delegates to the Supreme Soviet be 21 not 18 as proposed in the draft.

More radical proposals were discarded, Brezhnev said. Among those he listed were proposals for equal pay and pensions regardless of type of job or seniority; abolition of the vital private garde plots that account for a sizable share of Soviet market produce; and elimination of the 15 separate republics and of their theoretical right to secede, and replacing them with a unified state.

"I think the erroneousness of these proosals is quite clear," he declared.

Part of Brezhnev's derision was clearly aimed for effort at the Belgrade conference on the Helsinki agreemets on European Security and cooperation that opened today in the Yugoslav capital. The record of the Soviet Union on Human rights is sure to become a major and controversial part of that conference. Since early this year, the Kremlin has jailed deported and intimidated the small group of human-rights activists here and in other Soviet cities.

The new constitution includes a section entitled "The State and the Individual" where such basic rights as freedom of speech, assembly, worship and redress of grievances are set forth. Even as the Supreme Soviet was meeting, however Western reporters were told that some 50 Jewish "refusedniks" were being held under virtual house arrest by the powerful state secret police. They had intended to appear at the Supreme Soviet meeting to seek exit visas that have long been denied them and to speak of the social and economic ostracism to which they have been subjected.

But the assembly chamber, a large airy room inside the Kremlin walls, was peaceful as unanimous "ayes" were quickly registered at various procedural recommendations of the leadership. There were no "nays" and no "abstentions" recorded. Several foreign sources reported seeing Nikolai Podgorny among the delegates. Podgorny, 74, was ousted last June as president of the country, to be replaced by Brezhnev, but he still retains his seat as a deputy representing a Moscow constituency.

For Brezhnev, the session was a unique moment. He heads the Constitutional Drafting commission, the Central Committee, the Politiburo and the state His new constitution raises to a supreme position the party in which he has labored as his life. As he stood beneath a statue of Lenin, Brezhnev received the applause of the delegates - clapped himself in the Russian custom and perhaps permitted himself what appeared to an observer in a balcony several hundred feet away the slightest glimmer of a smile.