JOHANNESBURG, OCT. 10 -- Gunmen fired on a bus outside Durban Tuesday night, killing six persons and wounding the other 27 people aboard, police said today.

The attack, which police said was unrelated to an act of gang violence that occurred in Durban on Monday, raised fears that fighting between rival black factions could resume in Natal province, where a fragile peace has held since early summer after four years of violence that has claimed at least 4,000 lives.

Police said unidentified gunmen riding in a car opened fire Tuesday night on the bus near Kwamashu, a black township outside Durban. The motive for the attack was not immediately known, they said.

The gunmen, firing as their car overtook the bus, used Soviet-made AK-47 and South African-made R-1 automatic rifles in the attack, police said. They said all 33 victims were black.

Such seemingly random attacks in the past have been linked either to turf warfare between supporters of two rival black nationalist groups, the African National Congress and Zulu Chief Mangosuthu Buthelezi's Inkatha Movement, or to struggles between taxi and bus companies over routes. The two nationalist groups are vying for political primacy in the movement to gain a share of political power from South Africa's white-minority government.

Police said an attack by a black gang on whites near Durban's fashionable beachfront Tuesday morning was led by a religious zealot, Jepu Ngcobo, who believed he was the son of God and that 1990 was the blacks' "year of freedom" in South Africa. They said his group was called the Ninevitist Sect.

Ngcobo led a gang of between 12 and 20 youths he had recruited Monday night on a rampage after stealing knives from a martial-arts store.

Members of the gang stabbed eight whites before Ngcobo was shot four times by an off-duty policeman and the others fled into the crowd.

Some of the gang's members wore shirts printed with slogans of the Pan-Africanist Congress, a hard-line black nationalist group that has rejected power-sharing negotiations with the government. But police said they did not believe that the Pan-Africanists were involved or that the attacks were politically motivated.

Meanwhile, 21 political prisoners were released, as President Frederik W. de Klerk had promised Monday following the latest round of talks with ANC leader Nelson Mandela.

At least 60 blacks, most of them ANC members who had been imprisoned for politically related crimes, have been released since an Aug. 6 accord between the ANC and government on the freeing of political prisoners and return of exiles.

Those released today had been convicted of crimes ranging from possession of hand grenades to murder and treason and had been sentenced to terms of four to 16 years.

Reading a statement at a press conference in Cape Town on behalf of 15 ANC prisoners released from nearby Robben Island prison, former prisoner Vronda Banda called on "all peace-loving South Africans to support the {ANC} peace initiative" and on those who did not support the ANC "to give peace a chance."

Another released prisoner said there were about 200 other "comrades" still on Robben Island who expected to be released by April 30 in accordance with the ANC-government accord. An estimated 3,000 political prisoners are being held throughout the country.

The Associated Press added from Cape Town:

Following the prisoner release, the ANC issued a statement protesting the number of its followers that were left behind bars.

"All our members, whether in prison or exile, qualify to be released or to return to their country," the statement said. "We therefore call upon the government not to engage in useless, time-wasting, delaying tactics on this matter."