Leaders of the 50-nation Organization of African Unity opened a summit meeting here today at which hostility toward the Reagan administration's policy on Namibia was practically the only issue unifying them.

President Reagan's harshest critic was Namibian guerrilla leader Sam Nujoma who said at the opening session that the administration had "sabotaged" the U.N. plan for independence of the territory, also known as Southwest Africa, which is controlled by white-ruled South Africa.

Frequently using Reagan's name, Nujoma said the United States and South Africa had entered "an unholy alliance" and were subjecting Namibians to "cruel and unusual manipulation" by trying to set out in advance the terms of an independence constitution. This, he said, would result in a settlement on South African terms.

"The expanding dealings between the two countries since Reagan took over are, in our view, a very dangerous development which may engulf the entire continent of Africa in an East-West conflict," he said.

He also accused the U.S. administration of "hostility and insensitivity" toward the disenfranchised black majority in South Africa.

His criticism of the United States was far sharper than remarks he made earlier this month during a visit to Zimbabwe where he appeared to be holding out prospects for compromise. Since then U.S. officials have made friendly gestures toward South Africa and U.S. Deputy Secretary of State William Clerk has visited Namiba and held talks with South African-supported political forces there.

[The Angolan news agency Angop reported that Angola, the northern neighbor of Namibia which provides sanctuary for Nujoma's Southwest Africa People's Organization will press at the OAU meeting for a new international conference on the territory's status. A conference in Geneva to set the terms of Namibian independence failed to reach agreement last January.]

The direct attack by Nujoma, who has been leading a 15-year, low-key war for independence, was in contrast to the more muted diplomatic tones of African foreign ministers over the last 10 days as they prepared for the summit. African officials, however, have made clear their unhappiness with the direction of Reagan's policies.

The new chairman of the OAU, President Daniel arap Moi of pro-Western Kenya, echoed Nujoma's attack without mentioning the United States or Reagan by name.

He criticized the efforts of five Western nations -- the United States, Britain, France, West Germany and Canada, known as the contact group -- in trying to bring about a settlement.

"The so-called contact group of Western powers seems to have lost proper contact and there are dangers of losing time through initiatives outside the framework of the United Nations," he said.

This was a reference to U.S. efforts to make alterations favored by South Africa in the U.N. resolution designed to bring about independence. South Africa has refused to implement the plan although it agreed to it in principle more than three years ago.

He called for withdrawal of the 20,000 South African forces fighting Nujoma's guerrillas in Namibia and said that until South Africa agrees to independence for Namibia Africa has no choice but to support armed struggle.

Moi, who probably feels the need to distance himself somewhat from Washington to bolster his nonaligned credentials while chairman of the OAU, also indirectly criticized the warming U.S. relationship with South Africa.

"We in Africa should now make it clear that those who support the racist regime in South Africa . . . are against us both individually and collectively," he said."There are no two ways about it."

Criticism of the United States has been one of the few areas of agreement in the preparations for the fourday summit being attended by about 30 African heads of state or government.

The most divisive issues are the Libyan military intervention in Chad, King Hassan's threats to break up the organization if it recognizes a nationalist group fighting for independence of Western Sahara -- claimed by Morocco -- and border disputes in both East and West Africa.

Even the site for next year's summit, the Libyan capital of Tripoli, is a subject of controversy.

Libya is a focus of much of the divisiveness in the OAU and a number of members wish to switch next year's summit away from the North African country to prevent its radical leader Muammar Qaddafi from becoming Africa's spokesman next year.

Outgoing OAU chairman, Sierra Leone President Siaka Stevens, gave a generally gloomy picture of the year in Africa, saying there was no end in sight for the war in Western Sahara and citing the Chad problem where Libyan troops intervended in a civil war.

The Libyan intervention, he said, caused "disturbing consequences for Chad and neighboring countries" and "has not met with general approbation." In an obvious reference to Libya he criticized those who "fan the flames of war in Chad."

On an ironic note, he voiced hope that the people of Uganda, long wracked by tribal violence, may find unity. It was reported this afternoon from Kampala that Uganda's West Nile province was once again the scene of clashes and that 55 persons had been killed at a mission by the Uganda Army.

In addition, Uganda was reported to be holding members of the Libyan Embassy in Kampala under house arrest.

No reason was given but the incident was just the latest evidence for a remark Moi made today in his speech.

"No one," he said, "can say Africa is truly united."