Brazilians are showing increasing concern at the growing dominance of the economy by this country's authoritarian military government, which justified its seizure of power 12 years ago partly as a move to halt socialist trends in Brazil's last democratic civilian government.

The rightist military leaders argue that the deep state involvement in the economy is needed because private firms - local and foreign alike - lack the money and resources for large-scale undertakings. Thus, they say, the state is simply "filling empty spaces" in the economy.

But private businessmen here are not convinced by this official rhetoric, and say they see no difference between creeping statism Brazilian-style and the centrally directed economies typical of Communist countries.

Furthermore, many Brazilians say that if the current trend toward state expansion in the economy continues, Brazil will find it increasingly more difficult to ever return to democratic civilian rule.

A detailed study by Visao magazine, a Brazilian publication similar to Business Week and Fortune in the United States, shows that of the 571 government and government-affiliated companies now existing in Brazil, more than 200 were founded after the 1964 military coup.

A Visao survey found that 78 of the 200 largest corporations in Brazil are controlled by the government and account for than two-thirds of the combined assets of all the companies on the list.

Of the top 30 corporations here, 24 belong to or are affiliated with the federal administration or state governments. Four of the six nongovernment companies are subsidiaries of foreign-based multinational firms!

Only two private Brazilian corporations are among the tip 30: a construction company, in 28th place, and a sugar-growers' cooperative, in 30th place.

J. C. de Macedo Soares Guimaraes, a retired Brazilian admiral and a frequent critic of the growing state influence in the economy here, warns that increased government economic control inevitably means increased government political control. He sizes up the situation bluntly:

"State capitalism cannot help but generate a closed political system. And when the dismantling of private enterprise is consummated, Brazil will find itself with a political regime that has no freedom and no justice - and whose name is none other than communism. What irony! We are establishing communism under the flag of anticommunism.

Macedo Soares was indicated last year on charges of "endangering national security" for allegedly slandering Brazil's federal planning secretary, whom he holds partially responsible for the current spread of statism. The case was eventually shelved.

Brazilian governments became involved in the economy long before the oresent military regime came to power. Brazil began nationalizing its railroads in 1901. Next, the government moved into banking and credit and the regulation of the overseas marketing of such Brazilian commodities as coffee and sugar. Brazil's first major industrialization project, a steel mill constructed in the 1940s, was built with government capital, because local private businessmen and foreign corporations could not or would not commit the millions of dollars that were needed.

Later, due in large part to out-and-out nationalism, the Brazilian government went into the oil-prospecting and refining business and into iron-mining.

Ever since the 1964 military take-over, however, the proliferation and expansion of government-run companies here has become, in the eyes of many, a monster out of control.

For example, the Banco do Brasil, the government-run commercial bank, recently opened 251 new branches throughout the country, raising its total to more than a thousand. Private Brazilian bankers complain bitterly that the government is freezing them out of scores of cities and towns where they could do a perfectly adequate job of lending money and taking deposits.

Petrobas, the government oil company, now has 35 subsidiary companies including a fertilizer plant, a synthetic rubber company, and an overseas trading company - which, among other things, has been selling shoes to the Soviet Union.

Electrobas, the government's electric-power holding company, has set up 37 affiliated firms in recent years, and in only 19 months since the signing of a controversial nuclear technology agreement between Brazil and West Germany, Nuclebras, the Brazilian government's nuclear-energy company, already has six subsidiaries and two more on the way.

President Ernesto Geirsel, a politically moderate ex-army general who says he is not opposed to steering Brazil slowly back to civilian government, has also said that his regime will do everything possible to strengthen and develop local private enterprise.

The influential newspaper O Estado de Sao Paulo editorialized, however, that while the president's objectives may be laudable, the rate of expansion of state enterprises in the Brazilian economy since the military coup has turned out to be "noxious to the nation's overall interests."