JOHANNESBURG, JULY 6 -- A powerful bomb exploded early today at a crowded black bus and taxi station here, injuring 27 people, two of them seriously.

No one claimed responsibility for the bombing, but white extremists have threatened to resort to violence to protest President Frederik W. de Klerk's commitment to negotiate a new constitution with black nationalists. If the attack turns out to be the work of white extremists, it would be the first major action by white, right-wing terrorists in South Africa.

There have been several bomb explosions throughout South Africa this week, but today's bombing was the first one targeted specifically at people.

It drew an angry reaction from the African National Congress, which accused the government of being soft on white extremists and failing to respond adequately to their threats.

"The ultra-right and neo-fascist and paramilitary forces are on the rampage, emboldened by the authorities' extremely permissive attitude toward them," the ANC said in a statement.

"The ANC demands that the authorities disarm and disband the ultra-right groups and other neo-fascist organizations promoting racial violence against our people," the statement added.

The bomb clearly appeared aimed at blacks, since the downtown station is used exclusively by commuters from the black townships.

Witnesses said the bomb exploded during the rush hour at 8:25 a.m. as thousands of black workers arrived at the station in buses and vans.

They described seeing people hurled through the air by the blast and many people cut by shards of flying glass. At least seven taxis and three private cars were destroyed. A nearby shopping center was extensively damaged.

Most of the injured were women. They lay in the street and on sidewalks waiting for ambulances while police cordoned off the area.

Nhlanlha Mbatha, a journalist, said he was standing in line waiting for a bus when the bomb exploded in a concrete trash can 50 yards away.

Soon after the explosion, a caller describing himself as a member of the White Wolves extremist group telephoned a liberal newspaper, the Daily Mail, to say it was next in line for an attack.

The National Union of Mineworkers, largest of the black labor unions, said its members had found and defused a bomb at its offices in the mining town of Oberholer, west of Johannesburg.

Later today, police searched the home of Robert van Tonder, leader of a right-wing organization called the Boer State Party, some of whose members have openly advocated violence and claimed responsibility for several bomb attacks.

Police removed documents from the farmhouse, west of Johannesburg, but did not arrest van Tonder.

An angry van Tonder said the search was further proof that "democracy is dying in South Africa" as the government pursued its "persecution-mania against Boers, Afrikaners and rightists."

In another development today, Law and Order Minister Adriaan Vlok said police arrested three guerrillas and an accomplice under instructions to kill so-called "warlords" of Chief Mangosuthu Buthelezi's Inkatha Movement in Natal province.

Vlok said the men had identified themselves as ANC members.

Speaking at the opening of a new police college, Vlok said they were carrying grenades when they were arrested. According to Vlok, they confessed to the scheme and showed police an arms cache containing automatic rifles and ammunition.

Rival Zulu supporters of the ANC and the Inkatha Movement have been locked in a bitter civil war in Natal for the past three years, with each side accusing the other of responsibility for the violence and bloodshed.