A bomb exploded in a South African resort shopping center near Durban today, killing six whites, including three small children, and injuring at least 61 others.

It followed the bombing of a vanload of vacationers in downtown Durban on Thursday that seriously injured a white family of six, indicating an apparent switch of tactics by insurgents of the African National Congress to attack previously avoided civilian targets.

As of late tonight, however, no group had said it was responsible for today's blast.

Meanwhile, Winnie Mandela, wife of the imprisoned ANC leader Nelson Mandela, was the center of more drama. A court released her without bail following her detention yesterday for defying a government order to leave the black township of Soweto.

She returned immediately to her house in Soweto after the court released her with a warning to appear Jan. 22 to face charges of violating the restriction order, which could lead to a three-year prison sentence.

At that point, police again surrounded her house. After a moment of tension, during which it seemed the police might forcibly remove her from the house for the third time in three days, Mandela's lawyers negotiated a temporary compromise, and she left the house to fly to Cape Town for a Christmas Day visit to her imprisoned husband.

Crowds had packed the Johannesburg magistrate's court for Winnie Mandela's appearance following her arrest for defying a relaxed restriction order that still required her to leave her Soweto home.

Crowds mobbed her in the street outside the court building after her release and followed her through the streets, chanting ANC slogans in a demonstration of support for the outlawed black nationalist movement.

Today's bomb explosion, in the popular seaside resort of Amanzimtoti, south of Durban, was the most serious attack since a car bomb killed 20 whites and injured more than 200 in Pretoria in August 1983. The ANC claimed responsibility for that explosion.

It is summer now in the southern hemisphere and thousands of white South Africans flock to seaside resorts for the holidays. They were swarming through a shopping center in Amanzimtoti, many dressed in shorts and beach clothes, when a powerful bomb reportedly planted in a trash can exploded.

The blast ripped through the crowded arcade, shattering the fronts of 14 shops. Shards of flying glass slashed into the Christmas shoppers. The dead included two women, two toddlers aged 2, a boy of 8 and a girl of 16.

Dozens of bloodied people were carried from the devastated shopping mall to a makeshift first-aid post on a sidewalk, where ambulances ferried them to hospitals in Durban, 30 miles away.

It was the third such attack in eight days. Thirteen white civilians have been killed. While there was no public claim of responsibility for today's attack, a government minister accused the ANC of carrying it out. President Pieter W. Botha, in a statement, denounced the "terrorist attack" and expressed sympathy for the victims' families and the survivors.

The ANC, which is committed to trying to overthrow white-minority rule by guerrilla warfare, generally has tried to avoid civilian targets and concentrated on attacking installations and military personnel.

As the black casualty toll soared during the current wave of violence in townships, the ANC reportedly came under pressure from young militants at a congress held in Kabwe, Zambia, last June to switch to "soft" civilian targets.

This was resisted by the congress' older leaders, notably its president, Oliver Tambo. Eventually, according to reports, a compromise was reached in which the guerrillas were instructed to continue to concentrate on military targets but to have fewer scruples about civilians who might be killed in "cross fire."

Tambo stated specifically in an interview published here last month that the ANC would not deliberately bomb "supermarkets and cinemas."

Observers believe the latest attacks against "soft" targets in the Durban area indicate either that the ANC leadership has given the go-ahead for a more ruthless campaign or that militant young cadres are taking matters into their own hands.

Louis le Grange, the minister of law and order, who interrupted his seaside vacation to visit Amanzimtoti today, said the attack showed conclusively that the ANC was now committed to a campaign of terrorism against civilians.

"This places a serious moral responsibility on civilized western governments to condemn those responsible," le Grange said.

Meanwhile, the black death toll in South Africa's year of violence continued to soar, with two more persons shot to death when police opened fire on a group of flag-waving marchers in Soweto today.

Yesterday two blacks were killed by police gunfire in a tribal area called Mutse, 100 miles north of Johannesburg.

This brought to more than 960 the number of blacks killed since September 1984. Today's bomb blast brought the white death toll to 19.

The Johannesburg homes of two prominent Asian civil rights workers were gasoline-bombed yesterday, adding to the overall picture of a rising spiral of racial violence.

Sheena Duncan, president of the widely respected Black Sash civil rights organization, expressed concern at the rate at which South Africa's political conflict was approaching a state of civil war.

"There is a real danger that the current reliance by all groups on weapons of war -- guns, gasoline bombs, limpet mines, a powerful Army, uncontrolled security police powers and violent repression -- will lead us into an endless destructive hopelessness in a Beirut-type conflict in which there is no justice, no truth, no peace and no freedom from fear," Duncan said.

She added that unless President Botha committs himself urgently to dismantling the apartheid system of segregation, she could see no hope of peace for South Africa.