South Africa has disclosed that its military security establishment has taken a leading policy-making role in the government.

Two senior officials gave details at a press conference here Wednesday of an extensive organization called the National Security Management System, headed by a powerful State Security Council, that helps formulate policy on a wide range of issues not normally associated with defense and security. These issues include economic, political, labor, press, social, sport, cultural and religious matters.

The basis for this sweeping range of activities is an assessment by the security chiefs that South Africa is the target of a full-fledged offensive directed from Moscow.

The officials said the offensive is seen as being waged on many fronts through local surrogates. To counter it, they said, a national strategy has to be devised covering all areas in which the enemy is thought to be active.

Ironically, the disclosures came in an attempt by the government to deny, or at least play down, recent claims that the military has virtually taken over the running of the country.

The claims appeared in two academic studies published last month--one by an American professor of politics, Kenneth W. Grundy, who spent a year at the Institute of International Affairs in Johannesburg, and the other by two local political scientists, Deon Geldenhuys and Kennie Kotze, of Rand Afrikaans University.

The Geldenhuys-Kotze study describes the State Security Council as an inner cabinet that sits under the chairmanship of Prime Minister Pieter W. Botha. The prime minister then presents the council's decisions to the full Cabinet as a fait accompli, according to the study.

Grundy likens it to the Politburo of the Soviet Communist Party--the real decision-making body--with the Cabinet, like the Soviet Council of Ministers, a rubber-stamp and policy-coordinating body.

Grundy, who is from Case Western Reserve University in Cleveland, also suggests that the military's increased influence coming through the council has resulted in the South African government adopting an interventionist policy toward neighboring black states and seeking to delay a settlement in Namibia.

Apparently worried that these claims might strengthen accusations that he is trying to establish a military dictatorship through a new constitution that is due to be put to a whites-only referendum Nov. 2, Botha ordered the secretaries of the council and the Cabinet to explain the workings of the system at Wednesday's press conference.

It was a rare disclosure. The South African government seldom holds high-level press conferences--although Botha held one last week to deny that the new constitution will give him dictatorial powers--and it is normally reluctant to reveal details of its inner workings, especially on security matters.

Both the council's secretary, Gen. Andre J. Van Deventer, and Cabinet secretary Johannes P. Roux contended that the council is just one of four advisory committees to the Cabinet. They said it has no executive authority of its own and all its policy recommendations must be approved by the Cabinet before they can be implemented.

In the briefing, however, the two secretaries made it apparent that the council is much more influential and its activities are more wide-ranging than the other Cabinet advisory committees.

Grundy contends that the council is unlike the other committees since it is chaired by the prime minister. This means, he said, the Cabinet is unlikely to reject its recommendations, knowing they already have its chief's approval.

Asked about this, Van Deventer said the Cabinet did sometimes refer recommendations back to the council for further study or more information, but he would not say how often this had happened.

The two secretaries implicitly confirmed a contention by Grundy that virtually all foreign or domestic matters can become a subject for the council to examine.

Explaining the concept of the ubiquitous enemy who must be combatted at all levels, Van Deventer said: "All possible methods are used by the enemy , with armed combat only the last resort. This implies that the defender must ensure preparedness in many fields other than the military."

A particularly important area, he said, was the "war of words" against South Africa.

The National Security Management System was instituted to achieve complete participation by all government departments to reach this state of total preparedness, Van Deventer said.

In addition to the prime minister, the State Security Council is composed of the most senior Cabinet minister after him (currently Manpower Minister Stephanus P. Botha), the foreign, defense, police, and justice ministers, the chief of the defense force, the commissioner of police, the secretary for security intelligence and the directors-general of justice and foreign affairs.

The council has a working committee and a secretariat headed by Van Deventer. The secretariat's four branches handle intelligence interpretation, coordination and formulation of strategy proposals, preparation of advice on how to counter the war of words and administration.

Under that are 15 committees responsible for policy planning in specific fields. Finally there are nine joint management centers serving different areas of the country, whose job, Van Deventer said, is to carry out the strategy decisions at the regional level.

Van Deventer and Roux would not comment on specific policy decisions by the council, but Grundy suggests it has eclipsed the Foreign Ministry in devising a more aggressive regional strategy in southern Africa.

Grundy attributes to the council what he calls a policy of preemptive intervention, including cross-border raids on members of the banned African National Congress in Maputo, Mozambique, and in Maseru, Lesotho, in 1981 and 1982, and also of coercive hostility toward governments already inclined to be hostile to Pretoria.

He suggests that South Africa may have adopted an interventionist policy in the region as the result of council members with a military orientation making narrow and inaccurate readings of regional politics.

Grundy also contends that the increased military influence in foreign policy has reduced the prospects of an early settlement in Namibia (South-West Africa). The defense force possesses a virtual veto over any settlement proposal, he says.

Grundy traces the growth of South Africa's defense establishment, from a budget of $40 million and a standing force of 21,500 men, with 56,500 reserves, in 1960, to a budget of $2.78 billion and a standing force of 81,400 men, with 157,000 reserves, in 1982.

Conscription, backed by a campaign to glorify military service in schools and the press, has resulted in the militarization of the ruling white community, Grundy says.

The business sector has been militarized, too, he contends. Armscor, the state-owned arms manufacturing company formed nine years ago to counter a U.N.-backed ban on arms sales to South Africa, has grown into one of the country's top 20 companies with assets of more than $1 billion. It has made South Africa the 10th largest arms manufacturer in the world.

Noting that up to 60 percent of Armscor's production is contracted out to private companies, Grundy says this has resulted in many firms, including foreign firms, being co-opted into security collaboration.