South African forces launched a raid into southern Angola today in what military officials said was a preemptive strike against Namibian guerrillas based there.

Gen. Constand Viljoen, chief of the South African Defense Force, said the "follow-up operation" was designed to prevent guerrillas of the South-West Africa Peoples Organization (SWAPO) from launching bombardments and ground attacks against northern parts of Namibia, the South African-controlled territory that abuts Angola.

He gave no indication of the size of the invasion force, but he said it was being supported by the South African Air Force. He said the operation was aimed primarily at SWAPO's 8th Battalion and other "special forces."

The battalion consists of between 400 and 800 men, a military spokesman later told the state-run national television network. Lt. Gen. Ian Gleeson, chief of staff of the South African Defense Force, said the action would "hopefully . . . be over within the next week."

Viljoen said the Angolan military had been informed of the incursion and "requested not to interfere." There was no immediate comment from the Marxist government in Luanda.

South African incursions into Angola have occurred several times in recent years. It was unclear whether today's strike went far enough north to risk encountering Cuban troops, but the possibility appeared unlikely, given the known Cuban deployment in the south. There are about 25,000 Cuban troops in Angola in addition to Soviet and East European advisers.

The incursion could mean a new setback to American hopes for a negotiated settlement between South Africa and Angola leading to a withdrawal of the Cubans from Angola and to independence for Namibia. American officials in Pretoria told journalists at a briefing before the announcement of the raid that they were "keen to hear" a response from Pretoria to Reagan administration proposals made early this year to revive the stalled talks between Luanda and Pretoria.

Viljoen's brief statement tacitly acknowledged the potential diplomatic damage of the raid, saying his forces had had no alternative following "irrefutable evidence of SWAPO's plans" and the movement's "contempt" for repeated South African warnings to cease its attacks.

He said the strike followed months of "intensive intelligence-gathering" inside Angola showing that SWAPO "planned standoff bombardments on military bases and attacks on soft targets" inside Ovamboland, Namibia's northern region and major population center.

The guerrillas also intended to attack "larger towns and residential areas," according to Viljoen, and had increased "abductions, intimidation and sabotage" sharply in preparation for new attacks..

The intelligence was confirmed by the capture of two guerrillas inside Namibia within the last 24 hours, the military spokesman said.

The last major cross-border operation by South African forces into Angola took place in late June and lasted three days. South Africa reported killing 57 guerrillas and capturing five during the action, in which its troops reportedly moved 25 miles into Angolan territory. Angolan forces apparently made no attempt to repulse the troops.

From 1981 until earlier this year South Africa maintained a permanent military presence in southern Angola to try to thwart guerrilla attacks by SWAPO, which has fought a 19-year bush war to free Namibia from South African control.

Annual military operations by SWAPO into Namibia, usually commencing with the rainy season sometime between September and November, are regular events.

In December 1983 South Africa launched its largest cross-border strike against SWAPO, an operation that for the first time prompted a large-scale military response by Angolan forces backed by Cuban troops and Soviet arms and advisers. Early last year South Africa and Angola agreed to a cease-fire and a gradual disengagement of forces that U.S. diplomats hailed as a major step toward a settlement.

South African forces gradually pulled back to the Namibian border during the course of more than a year. But the two sides remained far apart over a timetable for Cuban troop withdrawal, and the agreement unraveled when two South African commandos were killed and one captured inside Angolan territory last May and South African troops reentered southern Angola in June.