JOHANNESBURG, JULY 30 -- A car bomb described by police as the most powerful terrorist device ever used in South Africa wounded 68 people today when it exploded downtown outside a military headquarters.

A reporter for the government-supervised television news said police estimated that the bomb, which broke windows in a five-block area, weighed about 180 pounds. Three Cabinet ministers accused the African National Congress guerrilla organization of planting it.

Tom Sebina, spokesman for the ANC in Lusaka, Zambia, said he had no immediate comment. "We have not yet heard from our people in South Africa," he said.

The television report said the bombing marked the 66th anniversary of the founding of the South African Communist Party, outlawed by South Africa in 1960 along with the ANC. The government says the ANC is dominated by Communists.

The ANC claimed responsibility for a car bombing here May 20 that killed four policemen and injured 14 other people.

Defense Minister Magnus Malan called today's explosion a "deed" of the South African Communist Party "through their stooges the ANC." Malan, quoted on television, also warned South Africa's black-ruled neighbors against "harboring terrorists."

Government-run Radio South Africa said earlier a bomb in the back of a delivery truck caused a floor to collapse inside the 11-story brick building of the South African Defense Force.

Minister of Law and Order Adriaan Vlok blamed the ANC. He said the fact that half the injured were black gave the lie to the ANC claim that its target is the country's white minority. Minister of Justice Hendrick Jacobus Coetsee told Parliament the ANC claimed responsibility, but he did not say what he based this on.

A police spokesman, Col. Frans Malherbe, said the bomb left a three-foot-deep crater in the street and was "the heaviest bomb yet in Johannesburg." There have been 11 explosions in the city this year, and none of the devices detonated outside Johannesburg has rivaled today's for power.

Malherbe said soldiers and children were among the injured, which included 29 whites, 32 blacks and seven of mixed-race. Malherbe said four whites remained hospitalized, in satisfactory condition, but he did not have figures for the other races, who went to a different hospital.

The bomb destroyed or damaged 20 vehicles. It sent a large piece of metal from the delivery truck into a tree 500 yards away. Hundreds of bricks spilled onto the sidewalk, mixing with smoldering chunks of metal.

Radio South Africa reported 20 buildings were damaged.

President P.W. Botha issued a statement in Cape Town calling the attack "a dastardly and callous act by terrorists. We reject with contempt the diabolical methods of people under control of godless communist forces."

The Citizen, a pro-government Johannesburg daily newspaper, reported today that the ANC recently repeated its long-standing call to take the fight into white areas of the country.

The ANC is the largest group fighting apartheid in South Africa. By law and custom, apartheid establishes a racially segregated society in which the country's 25.6 million blacks have no vote in national affairs. The 5 million whites control the economy and maintain separate districts, schools and health services.