Rhodesian forces smashed two black nationalist bases in Mozambique last week, killing at least 1,200 guerrillas and destroying large quantities of war material, the government said today.

The most devastating raids Rhodesia has conducted across the border came as Prime Minister Ian Smith was making plans to meet with moderate black leaders inside the country to discuss eventual majority rule.

The military command said ground and air forces launched the five-day operation Wednesday against the guerrillas' main base camp at Chimoio, 54 miles across the border, and a second camp near Tembue, in Mozambique's northwestern province of Tete, 132 miles inside the neighboring country.

One Rhodesian solider was killed and eight were wounded, the government said, The raids.The raids, the fourth operation into Mozambique officially acknowledged by Rhodesia, were conducted "in the interests of self defense, the military command's communique said.

Large quantites of war materiel including weapons, vehicles, ammunition, fuel, buildings and documents were destroyed," it said.

UPI correspondent Tony Avirgan, who was in Chimoio at the time of the attack on Wednesday, reported that Rhodesian jet warplanes roared over the town and dropped bombs on the guerrilla base camp 18 miles away. The jets were followed by groups of helicopter gunships, which raked the camp with machine gun fire.

According to guerrillas who survived the raid, troops alighted from helicopters and began systematically killing residents of the camp.

Since there is no effective communication between Chimoio and Mozambique's capital, Maputo, Avirgan's report was delayed five days.

The bombing went on for two days, and was clearly visible from Chimoio. The guerrilla refugees said the Rhodesians occupied the camp for two days. A column of Mozambican troops tried to fight its way to the base, but was driven off by aerial bombardment.

By the evening of the first day, hysterical and wounded survivors of the attack began streaming into Chimoio, which has a rudimentary hospital.

The Mozambican column finally reached the camp after the Rhodesians had left. Members of the relief force reported finding 100 bodies there, among them boys and girls in their early teens who had been receiving political indoctrination. Another 650 are receiving medical care at the hospital.

Officials say the full casualty toll of the raid may take weeks to determine, because the dead are spread out over a wide area of dense forests.

The corpses now are being bulldozed into a common grave.

Rhodesian intelligence sources said both camps were strategic bases for guerrillas of Robert Mugabe's Zimbabwe African National Union, largest of the two guerrilla armies fighting to topple Smith's white minority government.

Combined Operations Minister Roger Hawkins said Rhodesia had acted in self-defense against a buildup of guerrillas in the base camps.

Officials believe Mugabe has about 9,000 men, more than 3,500 in Rhodesia and the rest in Mozambique. If 1,200 guerrillas were killed in the raid, it would mean he has lost nearly one-fifth of his forces.

The second guerrilla army, the Zimbabwe African Peoples' Union headed by Joshua NkomoM is estimated to have about 3,000 men, several hundred inside Rhodesia and others in neighboring Zambia.

The government announcement of the twin raids electrified the white population of Rhodesia. Whites began trading enthusiastic congratulations and praising "our troopies" as soon as the military command broke its silence on the operations, which were reported by Mozambique earlier in the week.

Mozambique closed its 800-mile border with Rhodesia in March 1976, and since that time hostilities between the countries have increased steadily as the Maputo government stepped up its support of the guerrillas.

The military command said Rhodesian troops had not clashed with Mozambique's regular troops.

In the first announced raid against Mozambique in August 1976, Rhodesian troops destroyed what they called a terrorist camp 25 miles inside Mozambique, killing 340 persons, including 300 guerrillas. Mozambique said it was a refugee camp an reported that as many as a thousand people were killed.

In other raids in October 1976 and again early this year, few guerrillas casualties were reported, but troops brought back truckloads of arms and ammunition and said they destroyed tons of equipments before withdrawing.

Smith said last week that he agreed to the principle of one-man, one-vote elections, a major demand of the nationalists, in return for guaranteed safeguards for the country's 268,000 whites. The 6.5 million blacks now have limited representation in the government.

Bishop Abel Muzorewa, the Rev. Ndabaningi Sithole and Chief Jeremiah Chirau, the three moderate Rhodesia-based nationalist leaders, have given qualified acceptance to the prime minister's proposed talks. Initial discussions were expected sometime next week to lay the groundwork for a formal constitutional conference leading to elections.

The white population has held political power since the country became a self-governing colony of Britain in 1923. In 1965, Rhodesia broke away from Britain on the issue of majority rule.

Observers believe the three local black nationalist groups have the support of more than 80 per cent of Rhodesia's blacks, but they do not have direct control over the guerrillas, whose leaders have called the Smith plan a deceit.