Brazil marks 15 years under military rule Saturday, with brand-new President Gen. Joao Baptista Figueiredo's recent inauguration-day promise-to "make this country a genuine democracy" in which "the enjoyment of all human rights is fully assured"-still ringing in its ears.

Over the years, Brazil's 120 million citizens have grown used to hearing such hopeful and optimistic words from their leaders. The military seized power in a coup here on March 31, 1964 as a "Temporary" measure designed to "protect" democracy. Each of Figueiredo's four predecessors has paid lip service to the idea of an eventual return to democracy.

Public sentiment in favor of such a restoration is now stronger than ever. But it is a sign of the profound changes wrought in Brazilian society by 15 years of authoritarian rule that even those Brazilians who do not doubt the new president's sincerity believe that it will be more difficult than ever before for Figueiredo to make good his pledge during his six-year term.

"we are at a dead end, trapped, and so is Figueridi, even if he doesn't know it," said a former Cabinet minister now deeply disillusioned with the military-technocrat coalition that weilds power here. "We all realize that the system does not work and never really did. But there's no way for any of us to start all over again."

Publicly at least, Figueiredo has said nothing that would indicate the regime recognizes it may have reached a crisis point. The chief plank in his platform is to continue the process of gradual political liberalization begun by former president Gen. Ernesto Geisel.

Critics of the regime point to a host of political, social and ecomonic problems that have successfully been shunted aside for years but which are almost certain to be exacerbated by the liberalization process. They also find fault with the way political liberalization itself was instituted.

"The government simply decreed that there would be an 'opening up,' as if such a thing could simply be summoned out of thin air," and a Rio newspaper editor. "But nothing was done and nothing is being done to create the kind of environment in which such an 'opening up' can thrive.

"The political parties have been castrated, the old leaders have either died, gone into exile or had their political rights suspended, and no new leaders have been allowed to emerge," the editor added. "That is the real heritage of our 15 years of military rule, and now they suddenly announce a liberalization."

"The regime has created an entire generation of political illiterates," complained a 36-year-old banker, noting that he has never been able to vote in a presidential election. "Half our population is under 30, but they've been taught all their lives to stay away from politics, to leave such matters to the 'well-informed people."

Similar failings are also evident in other sectors of society. In the first major test of political liberalization, the government felt forced to intervene March 23 in a strike of 200,000 Sao Paulo metalworkers when unions and management were unable to come to a wage agreement after 12 days of negotiations.

"The Brazilian government is now trying to encourage collective bargaining, but Brazilian businessmen. Simply aren't ready," said a foreign diplomat in Brasilia last week. "They have relied on the government to handle this for 15 years, and they don't know how to negotiate."

Looking ahead, many observers see economic and social issues as the chief sources of future discontent with the regime. Unable for years to challenge the inequities built into the economic system installed in 1964, Brazilians are now taking advantage of political liberalization to express their dissatisfaction and demand basic changes in economic policy.

Brazil now has the 10th largest economy in the world and will soon surpass both Italy and Canada. Since 1964 Brazil increasingly has become a two-tiered economy, with a small, prosperous middle class in urban centers such as Rio and Sao Paulo and an impoverished mass in the countryside and industrial suburbs.

"The greatest problem Figueiredo faces is the unequal distribution of income. There are 120 million people here, but only 25 million are really participating in the fruits of the economic system," said a foreign diplomat.

The Figueiredo government has, in fact, declaced that one of its top priorities is to narrow the gulf between rich and poor. But it has also announced that fighting inflation, which is expected to reach 6 percent in March alone, is its chief worry.

"Brazilians ae in a sense victims of their own success," said a banker. "The economy has grown so much that its now too big to be controlled from the center. Figueiredo is going to have to decentralize if economic and social growth are to continue.