President Nikolai Podgorny of the Soviet Union was officially welcomed to Tanzania today by President Julius Nyerere, marking the first state visit by a top-ranking Soviet leader both to this country and the explosive southern Africa region.

His two-week African tour, which includes Tanzania, Zambia and Mozambique, is widely regarded as a clear indication of the growing importance that the Soviet Union attachs to this part of the world as well of its increasing influence both with the "front-line" states and that the African nationalist movements fighting to overthrow the white-ruled states of southern Africa.

But his formal arrival statement seemed aimed primarily at allaying the fears of many African states, including some the "front-line" ones, that the Soviet Union may have ulterior motives in seeking to expand its presence in this region of growing racial conflict.

Even before the Soviet president had descended from his Tupolo134 passenger jet, Soviet Union does not seek concessions or military bases or some special privileges, neither in African countires nor anywhere else."

The Soviet Union, President Podgorny assured his Tanzanian hosts, favors the "lessening of international tension" and the "elimination of hotbeds of conflict" to help young African states strengthen their political and economic independence."

The Soviet delegation is scheduled to have three rounds of officials talks with President Nyerere during its four-day stay here, stay here, the first one this afternoon.

THe 74-year-old Soviet president was welcomed to the Tanzanian capital by large crowds at the airport, along the highway into town and in the downtown area, but they showed distincly more curiousity than enthusiasm. By contrast, Cuban President Fidel Castro received perhaps a slightly smaller but far more emotional welcome when he arrived here last Thursday.

There was little shouting of Podgorny's name and only sporadic, polite hand-clapping as the visiting Soviet leader accompanied by 120 officials, went through the arrival ceremony.

Western sources here said that the Soviet leader's state visit had been under discussion for nearly 2 1/2 years but repeatedly put off a Nyerere's request.HIs trip stands in sharp contrast again to that of the Cuban leader, who came at his own request on a few days' notice to see Nyerere.

There has been speculation in diplomatic and press circles here that Castro's visit was, in effect, to pave the way for that of the Soviet president, but this seems unlikely given the risk of an unfavorable comparison for the Soviets between th receptions given the two Communist leaders.

[Castro, meanwhile, arrived to an emotional welcome in Luanda, Angola, where Cuban forces played a decisive role in the Angolan civil war, news agencies reported. Referring to the role of Cuban troops, Castro told Angolans that "it was possible to save your joys, your smiles, your future with the blood of the brave Angolan and Cuban fighters."]

Tanzanian officials have been signaling Western correspondents covering the Podgorny visit that the government views that Soviet Union as a rich superpower with little in common with poor Tanzania compared to Cuba. "Cuba is a very genuine friend. She has given everything possible to aid our development," said one Tanzanian source.

"We have never worked very closely with the Soviet Union," he remarked. "Their aid to us is negligible."

The official media here has also clearly reflected this difference in attitude toward the two Communist countries, devoting far longer and more effusive editorials to Cuba and Castro than to Podgorny and the Soviet Union.

Nonetheless, the Tanzanian Daily News made clear in today's editorial that socialist Tanzania regards itself as an ally of the Communist countries when it comes to fighting the "evils of colonialism and racism represented in the worst form as apartheid in South Africa."

At the same time, the local media has been stressing Tanzania's policy of non alignment and the need for the Communist states to avoid "unfair trade arrangements with the African socialist countries.

The Soviets seem only too well aware of their delicate position and this presumably is why they are anxious now to make a big diplomatic splash not only here in Tanzania but also in Zambia, President Podgorny's next step on his three-nation tour.

Soviet economic assistance to Tanzania is said to amount to only about $20 million while China has given an estimated $358 million and the United States, $177 million. The Soviet Union, however, is becoming increasingly important in the military field here.

It has provided Tanzania with a partial missile defense system advanced Mig aircraft and just recently, some medium tanks. Indeed, it seems well on its way to displacing China as this country's major arms supplier. Western diplomatic sources here sayd it is not clear whether the Soviet Union has sold these weapons or its providing them on credit.