Rhodesian warplanes bombed a crowded black nationalist camp 22 miles west of here and first reports were that there were heavy casualties.

It was the heaviest raid against a nationalist installation since Rhodesian guerrillas of Joshua Nkomo's Zimbabwe African People's Union shot down an Air Rhodesia Viscount airliner Feb. 12, killing all 59 persons aboard. Nkomo's guerrillas are based in neighboring Zambia.

Long convoys of trucks and ambulances rushed the dead and wounded through the afternoon and into the night from the camp to Lusaka's main hospital, which was cordoned off by troops and police. No specific casualty report was immediately available, but witnesses reported that hundreds were killed and wounded.

Lusaka hospitals issued an urgent call for blood donors, supporting reports of heavy casualties.

A Rhodesian military spokesman in Salisbury said the attack on the Nampundwe camp, which housed an estimated 14,000 men, women and children, was a "routine mission" and that the major retaliation for the downing of the airliner was yet to come. The Zambian government said the camp was not a guerrilla outpost, but a refugee camp.

Rhodesia said the raid was part of its continuing effort to prevent black nationalist guerrillas from sabotaging the April 20 election that is supposed to result in qualified black majority rule in Rhodesia for the first time.

Informed sources suggested that similar attacks can be expected against camps of the Patriotic Front guerrilla alliance in both Zambia and Mozambique in the weeks leading up to the election.

Zambia has been bracing itself for retaliatory raids from Rhodesian forces since Nkomo's guerrillas claimed responsibility for the surface-to-air missile attack on the airliner. The guerrillas apparently downed the plane in the mistaken belief that the Rhodesian military commander, Lt. Gen. Peter Walls, was aboard.

Last September, Nkomo's forces shot down another Air Rhodesian Viscount, resulting in the death of 48 persons, 10 of them reportedly killed by guerrillas after surviving the crash.

In a series of retaliatory raids beginning six weeks later, about 1,000 persons were killed. Rhodesian jets bombed one camp just 12 miles north of Lusaka on Oct. 19, killing more than 230 persons and wounding at least 600.

Witnesses to today's attack said four Canberra jet bombers first swept over the camp at about 2:15 p.m., dropping many bombs. The bombers were chased off by Zambian Air Force jets, but returned again to drop another bomb at 4:55 p.m., the witnesses said.

A brief Rhodesian communique said only that "all aircraft returned safely to base" following the raid. Nationalist and Zambian government sources said two Rhodesian planes were shot down and their crews killed.

Sources here said the Nampundwe camp housed elderly men, women, children and a group of young men recently arrived from refugee camps in Botswana.

The arrivals from Botswana had been airlifted at the expense of the Botswana government and most of them were scheduled to be moved soon to guerrilla training bases in Zambia and Angola, the sources said.

The roar of the British-built Canberras echoed through the streets of Lusaka. Police and soldiers cordoned off the university hospital and large crowds gathered in the streets.

The raid was the fourth reported cross-border bombing attack by Rhodesian planes in a week. On Saturday and again Sunday Rhodesians struck at Nkomo's camps around Livingstone, across Victoria Falls from Rhodesia. On Monday, the Rhodesians said they attacked Robert Mugabe's bases in Mozambique.

Rhodesia has now acknowledged about 20 major raids into Zambia or Mozambique in the past 2 1/2 years. They are known, however, to have made many more border crossings in pursuit of fleeing guerrillas.