A U.N. investigation of the fierce combat in Namibia found that nationalist guerrillas mounted a major incursion from Angola the day the territory began its transition to independence, Western diplomats say.

South Africa claimed the South-West Africa People's Organization fighters violated the U.N. plan under which it is ending its 74-year rule of Namibia.SWAPO leaders denied guerrillas involved in the fighting near Angola's border had crossed into Namibia and criticized U.N. administrators for agreeing to let South Africans reinforce police battling the guerrillas.

"Our people are being butchered under the U.N. flag," said SWAPO Foreign Secretary Theo-Ben Gurirab. He accused South Africa of initiating the fighting.

In Harare, Zimbabwe, SWAPO chief Sam Nujoma called Tuesday for the immediate deployment of more U.N. troops in Namibia. He spoke with reporters after meeting with diplomats from the United States and the four other permanent Security Council members - Britain, France, China and the Soviet Union.

Namibian police say at least 161 rebels and 19 police have died in the fighting that started Saturday, when a formal cease-fire was to have begun. They said the fighting continued Tuesday.

Under the U.N.-brokered independence accord, all guerrillas are to remain in Angola at least 100 miles north of Namibia's border until mid-May.

But diplomats said the SWAPO guerrillas apparently felt they were entitled under the accord to set up bases inside Namibia under U.N. supervision. The guerrillas have been fighting for 23 years.

The report on the situation by investigators led by the U.N. special representative for Angola, Martti Ahtisaari of Finland, was relayed to Secretary-General Javier Perez de Cuellar, who briefed Security Council members Monday, said three Western diplomats.

The diplomats said the report concluded that up to 1,000 guerrillas infiltrated northern Namibia but that their motive was unclear. Its details were not disclosed.

Black African diplomats were outraged. "There can never be any question of infiltration, these are Namibians," said Zambian Ambassador Peter Zuze.

South Africa has accused the rebels of infiltrating in an attempt to set up bases, an option that was rejected during 10 years of U.N. negotiations on terms of the Namibian independence plan.

U.N. diplomats said they believed many rebels misunderstood the terms of the cease-fire and thought they could safely march home bearing their weapons.

Gurirab insisted the guerrillas always had been in Namibia and criticized U.N. administrators for allowing South African troops to reinforce the territorial police.

Perez de Cuellar said the fighting did not mean the peace plan is failing, adding that most of the 4,650 U.N. troops planned for the mission are not yet on the scene.