South Africa experienced its second car bomb explosion in a week when a powerful blast destroyed vehicles and damaged buildings today, but caused no casualties, in the center of Bloemfontein, capital of the Orange Free State.

The Bloemfontein explosion underscored a rising cycle of violence and reprisal here.

Last Friday a car bomb explosion in a busy Pretoria street killed 18 people and injured more than 200. It was the worst such incident in South Africa's history, and marked a turning point in the black underground's insurgency struggle against the country's white-minority government.

Previously the African National Congress, which is spearheading this struggle, followed a policy of trying to avoid civilian casualties.

In acknowledging responsibility for the Pretoria bombing, the ANC's president in exile, Oliver Tambo, admitted to reporters in Dar es Salaam, Tanzania, Monday that this policy had now been changed.

The bombing resulted in a retaliatory air raid by South Africa on Monday against what it said were ANC insurgent camps in neighboring Mozambique.

Western governments are worried that the escalating struggle is not only going to intensify inside South Africa, but that it could engulf the whole region and perhaps draw in communist forces to bolster South Africa's jittery black neighbors.

The danger, revealed in the rhetoric accompanying Monday's reprisal, is that the government here is convinced the insurgency threat emanates from across the country's borders and not from inside. Its retaliation is therefore likely to be directed with increasing ferocity at its black neighbors as the ANC intensifies its attacks and directs them more toward "soft" civilian targets.

For South Africa, such retaliatory action is as much an emotional necessity as a practical answer to the insurgency threat.

Like Israel, South Africa's white population of 4 million is so small that when there are heavy casualties, as there were Friday, almost everyone knows somebody who got hurt.

The emotional trauma is very great and the demand for revenge imperative. The Afrikaners, who are the majority group among the minority whites, also share with the Israelis an Old Testament belief in swift retribution, an eye for an eye. They greatly admire and openly emulate Israel's military methods.

Although officials proclaimed Monday's retaliatory raid a military success, two Johannesburg political scientists, Tom Lodge of Witwatersrand University and Ian de Vries of the Rand Afrikaans University, believe the main motive for the raid was to placate white opinion.

"There was a psychological need to hit back," Lodge said.

"It was to calm white feelings," de Vries agreed.

This psychology points to the risk of an escalating cycle of emotionally charged attacks against civilians and increasingly furious retaliatory raids into neighboring countries.

Somewhere along the line the desperate neighbors are likely to seek military reinforcement wherever they can get it. That could mean the introduction of communist troops.

Mozambique might do as her sister former Portuguese colony, Angola, did and ask Cuba for troops. There are unconfirmed reports that Lesotho's prime minister, Leabua Jonathan, has been offered a military training team by North Korea.

The introduction of such forces would reinforce an obsessive belief in the minds of the South African government that it is the target of a communist-inspired "total onslaught," and would be likely to produce even more vigorous retaliation.

"I hope I'm wrong," said John Barratt, director of the South African Institute of International Affairs, "but if Mozambique were to import Soviet Bloc soldiers and materiel, I think South Africa could start conventional warfare."

This forbidding prospect comes at an awkward moment for western, particularly American, diplomacy in southern Africa.

The Reagan administration needs to justify its policy of "constructive engagement" with South Africa by showing it can ease tensions in the region, reduce Soviet influence and remove the 30,000 Cubans in Angola.

To that end, U.S. diplomacy has concentrated on trying to get South Africa and Angola to agree to a negotiated end to the civil war in South African-ruled Namibia and to improve relations between South Africa and Mozambique. Progress was being made with both, but an escalating southern African conflict would wipe all that out.

The conflict has taken a long time to reach this point. It was in 1961 that the African National Congress, banned and decimated by arrests after police had shot and killed 69 black demonstrators in the township of Sharpeville the year before, decided that the politics of peaceful protest were no longer possible.

It went underground and formed a military wing called Imkhonto we Sizwe, Zulu for "spear of the nation."

There were a number of sabotage operations against power lines and the like, but South Africa's efficient security police soon cracked down on it with widespread arrests and detentions. For the next 10 years the Spear hardly showed itself.

Then, in 1976, black students clashed with police in Johannesburg's segregated township of Soweto. More than 600 young blacks were killed and many left the country to join the guerrilla forces. They formed a new and more militant element in the ANC.

But still the ANC's revolutionary war remained remarkably bloodless. The 100 bomb attacks during the past 42 months have been mainly against installations, government buildings after closing hours and, occasionally, a manned police station.

The Pretoria car bomb marked a change in this policy. Although the car was parked outside South African Air Force headquarters, it was in a busy street and the bomb went off during the late afternoon rush hour. Predictably, two-thirds of the casualties were civilians.

The turning point in the ANC's attitude seems to have come with a South African commando raid in the Lesotho capital of Maseru Dec. 9. The commandos killed 42 people who they said were ANC insurgents. Lesotho and the ANC claimed they were refugees and local citizens. The young militants are said to have argued that South Africa's willingness to kill refugees had eroded the moral objections to hitting civilian targets.

Evidence that their pressure was succeeding came at the funeral of the Maseru raid victims on Dec. 19, when ANC leader Tambo noted that the organization's followers did not understand its policy of restraint. "We shall move in the direction dictated by the wishes of our people," Tambo added significantly.

The line crossed, there will be no going back. Innocence has been lost on both sides, and South Africa's low-level revolutionary war is going to become bigger and bloodier.