Nigeria warned the Soviet Union and Cuba yesterday that "They should not overstay their welcome" to become a "new imperial power" on the African continent.

In a speech before the annual meeting of African leaders, Nigerian head of state Lt. Gen. Olusegun Obasanjo told the Soviet Union that Africa was "not about to throw off one colonial yoke for another."

The Nigerian leader was equally critical of the recent Western military intervention in Zaire saying "paratroop drops in the 20th century are no more acceptable to us than the gunboats of the last century were to our ancestors"

It was the first time Nigeria, the most populous and militarily the strongest black African state, has spoken out so forcefully against the growing Soviet and Cuban military presence. Western observers attached all the more importance to his warning since Obasanjo chose the Organization of African Unity summit meeting to deliver it, thereby assuring maximum publicity. "The Soviets should therefore see it to be in their interest not to seek to perpetually maintain their presence in Africa even after the purpose for which they were invited has been achieved," he said.

"This way they run the risk of being dubbed a new imperial power, as in deed they already are being called even by those with whom they have had long association," he said in an apparent reference to Egypt, Sudan and Somalia, all of which have cut 'their once close ties to the Soviet Union.

"Let the Soviets and their collaborators heed this timely counsel," he added.

The Nigerian leader's speech was carefully balanced in its two-way attack on both Western and Eastern involvement in Africa.

In a reference to the French-initiated and American-supported inter-African military force now stationed in Zaire's southern Shaba Province, Obasanjo said, "we totally rejected as an instrument of neo-colonialism any collective security scheme for Africa fashioned and teleguided from outside Africa" and serving the interests of any superpower.

He appealed to African leaders faced with internal dissension to resolve their difficulties without appealing to outside powers to bolster them in mutual defense pacts "based on colonial relations."

"We must not allow the East and the West to divide us and get us against ourselves under any guise," he added: what Africa needs most today is not arms or "sterile ideological slogans" irrelevant to African society but massive economic assistance.

Obasanjo's speech was repeatedly applauded and at the end President Julius Nyerere, who is known to share similar views, walked over to congratulate him.

Summit observers have been watching to see whether a kind of "third non-aligned force" will emerge here between those African countries heavily dependent on Western military assistance, like Zaire, Chad and Mauritania, and those just as reliant on Soviet and Cuban aid, like Angola and Ethiopia.

Nigeria would be in an excellent position to lead such a force. It has no foreign troops on its soil and is widely regarded as "progressive" in its foreign policy while being "moderate" in its political system.

Nigeria has good relations with Angola and Ethiopia as well as with most of the Western-backed West African states.

There are more than 45,000 Cuban troops and advisers now in 13 African countries and about 8,500 French ones in seven others.

The issue of foreign military involvement that is dominating the summit is a major preoccuption of most African leaders. Thirty chiefs of state and four heads of government have shown up here, the largest number ever assembled for the annual event since the organization was founded 15 year ago.

Indicatively, one of the African organization's 49 members has not shown up here, the Comoro islands, where a recent white mercenary led coup has led to the immediate isolation of its new leader, Abdallah Mohammed.

Last week, African foreign ministers asked the Comoro delegation to leave their meeting as a sign of general disapproval of the way Mohammed came to power. Bob Denard, a mercenary who fought in Zaire in the early 1960s is now serving as defense and security minister in the islands.

Mozambique's leader, Samora Machel, delivered a stinging attack on just such continuing Western military involvement in African countries, warning that those Western nations supporting Rhodesia and South Africa and allowing the recruitment of mercenaries "cannot be our allies."

At the same time, he praised the Soviet Union and Cuba for their assitance to the liberation movements and to Angola.

The other speech attracting attention here yesterday was that of Guinea's president, Sekou Toure, who is attending the annual summit for the forst time in 13 years after patching up his differences in March with two rival west African states, Senegal and the Ivory Coast.

A firm Marxist, Sekou Toure called for peaceful coexistence between african coturies of different political systems.

He also attacked the presence of foreign bases on African soil after long allowing the Soviets some facilities in his country. He called such bases "a permanent aggression" against both the host state and other African countries.

Sekou Toure surprised the summit by suggesting that South Africa might one day become a full OAU member. But he said it would first have to achieve political equality of the races an end to all discrimination.