Sudanese President Jaafar Nimeri today denounced the Soviet Union's presence in Africa as "new socialist imperialism." His opening-day speech at the 14th summit meeting of the Organization of African Unity underlined the determination of Africa's long-alient moderates to speak out against the policies of the continuing radical leaders.

Without mentioning the Soviet Union by name, Nimeri told the ideologically split 49-nation organization that "socialist immigration will only turn the continent into an arena for international conflict.

Nimeri shared the [WORD ILLEGIBLE] spotlight with Ugandan President Idi Amin, who [WORD ILLEGIBLE] appearance just one [WORD ILLEGIBLE] said he had [WORD ILLEGIBLE]

Resplendant in the [WORD ILLEGIBLE] uniform of an air officer, had [WORD ILLEGIBLE] indecked with medals and a pistol strapped to his wallet Amin was applanded resoundingly in the conference hall. It was his first appearance outside Uganda since an unsuccessful attempt to overthrow him on June 13.

The presence of 22 heads of state makes this the largest OAU authoring in years.

Nimeri, who came to power in 1971 with Communist help but who has turned increasingly against the Soviet Union and toward the West, told the conference: "We do not want to replace on imperialist with another. Let us stand in solidarily against this new invasion aimed at impending our march forward."

Alluding to Moscow's efforts to overthrow Marshal Tito of Yugoslavia after his 1963 split with the [WORD ILLEGIBLE] and to current Soviet attacks on the Spanish Communist Party's endorsement of "Eurocommunism," he said: "We have known its (the Soviet Union's) maneuvers in our own country. We have known what it has done in Europe - in Yugoslavia yesterday and with the Spanish Communist Party today!"

With the radical-moderate split apparently in mind, Nimeri also insisted that the OAU badly needs to reform itself. He also condemned as "immoral" the reluctance of many Africans to denounce their own shortcomings.

Such silence - apparently on such sources of embarrassment as the continuing massacres in Uganda and Ethiopia - is an easy way "to save ourselves from getting involved in these problems," but constitutes a serious threat to all Africa, he said.

Nimeri's attack was belived specifically aimed at Ethiopia. He and the Maxist Ethiopian government are at odds, and Sudan has openly aided armed insurgent movements in Ethiopia.

Although he did not reiterate his standard demand that the OAU move its capital out of Addis Ababa, Nimeri denounced Ethiopia's recent arms deal with Moscow.

Noting that the charter forbids such pacts with non-African powers, he warned of dangers inherent in such involvements.

"It is no secret that Africa has been divided into groups in a manner which invites the great powers to play with it," he said, recalling that the OAU was founded in 1963 to overcome a similar radical-moderate ideological split.

He castigated the superpowers for exploiting African divisions, but they "could not have done so if Africa had acted as one man," he said.

He was apparently referring to the silence of African moderates after the Soviet Union provided decisive aid to pro-Marxist forces in the Angolian civil war in 1975.

Nimeri spoke soon after Albert Bernard Bongo, Gabon's president and new O.A.U. chairman, keynoted the moderates' newfound combativeness by calling for a general "washing of dirty linen in public" at what he dubbed the "summit of reconciliation."

Most of the heads of state in attendance are French-speaking pro-Western moderates. Only prominent radical here is President Houari Boumedienne of Algeria.

Despite the temptation of some observers to call the four-day Libreville meeting "the moderates' summit," the radicals were fighting to defend their positions.

Symptomatic of their determination was an all-night debate pitting moderate and radical foreigner ministers, who originally were scheduled to finish their work Thursday.

Their still-unresolved discussions, which were to resume late tonight centered on moderate efforts to denounce the radicals for questioning the hitherto sacrosanct national borders inherited from European colonial powers.