Soviet President Nikolai Podgorny's extended trip to southern Africa, which begins next week, the first such visit by a top Kremlin official, is a dramatic gesture of Moscow's interests anf growing influence in that increasingly tense region.

Podgorny will head a delegation said to number 120 people for talks with leaders of Tanzania, Zambia and Mozambique and key figures in the black nationalist and guerilla movements in South Africa and Rhodesia. The Soviets are certain to get a warm reception.

Nowhere else in the world is Moscow as openly bellicose as in southern African conflicts. It declares frequently, as Podgorny did again the other day, that the Kremlin "will continue to give aid and support to the peoples of Zimbabwe (Rhodesia), Namibia (Southwest Africa) and South Africa who are selflessly fighting the racist regimes."

One of the main purposes of the trip therefore is undoubtedly to assess how that aid - particularly the military kind - can be effectively channeled. Western sources here say that Soviet military assistance so far has consisted of small arms and artillery, but as the fighting intensifies, especially in Rhodesian weapons may be offered.

When arrangements for the Podgorny trip were begun about a peacegful transition to majority rule in Rhodesia were still under way, chiefly through the Geneva Conference. The Kremlin was sharply critical of those negotiations.

It was thought then that Podgorny was being dispatched primarily to reassert Moscow's presence in the area, so that the Kremlin would not be left wholly outside the bargaining - as so often happened in the heyday of former Secretary of State Henry A. Kissinger's Middle East diplomacy. In southern Africa, as in the Arab-Israeli conflict, Soviet leverage is always endangered by the fact that it deals with only one side.

The Western effort to resolve the issues in southern Africa are now stalled, however. Once again the Soviet Union's longstanding identification with the local rhetoric of "struggle against the forces of imperialism, colonialism, neocolonialism, racism and apartheid" gives Moscow considerable cachet that the Podgorny trip is obviously meant to enhance.

The top Soviet leaders are in regular contact with black nationalist - Joshua Nkoma, leader of the Zimbabwe African People's Union was just in Moscow for a weeklong visit, his second since last summer - so that the arduous trip by the 74-year-old Podgorny must have considerable symbolic and ceremonial significance in the Kremlin view.

Soviet backing for national liberation movements is a fundamental tenet of Kremlin ideology - an essential element in the Kremlin's policy toward all Third World countries - and it is in southern Africa where that backing can most vividly be demonstrated. Hence Podgorny's journey.

Podgorny will not be going to Angola, where Kremlin support for a victorious Marxist faction in the country's civil war last year gave the Soviets probably their greatest African success: Western analysts say that relations lith Angola are already so extensive that nothing further is necessary now.

On Monday the Soviets ratified a friendship treaty with the Angolans signed during a visit here last fall by Agostinho Neto, the Angolan leader.

"The treaty," said Podgorny in a speech given extensive coverage here "portrays in concentrated form our country's course in relationships with new countries in Africa . . ." Specialists interpreted this as meaning Moscow regards its ties with Angola as a model for those with other governments - present and future - in the region.

Podgorny will go first to Tanzania, site fo China's most elaborate African aid project, the Tanzam Railway. Tanzania's relations with the Soviets have been improving - Moscow now provides military assistance and recently announced plans to aid in the building of a hydro-electric plant.

The meeting will be first between President Julius Nyerere and a top-level Soviet official.

Although Zambian President Kenneth Kaunda has visited the Soviet Union, his relations with the Kremlin are not especially close and were strained by his criticism of Soviet involvement in Angola. A multi-million dollar credit in rubles extended to the Zambians in 1974 has not been used. Podgorny is likely to meet in Lusaka, Zambia, with nationalists from Namibia and South Africa.

Mozambique may well be the most important stop for Podgorny. President Samora Machel has proven to be an independent-minded personality whose devotion to Marxism-Leninism has thus far no extended to the sort of links with the Kremlin that Angola has forged.

Podgorny may be hoping to get a friendship treaty with Mozambique like the one Neto signed, but Western diplomats here say something more general and impressive is likely.

Moscow's strongest card in Mozambique is its commitment to the Rhodesian guerrilla war and Podgorny'trip indicates that will be fully exploited there.