Six blacks were killed by police gunfire today and at least 33 were wounded, authorities said, as rioting worsened in a South African city that had not been troubled by previous racial unrest.

The deaths came in a second day of rioting in black suburbs of Uitenhage, near Port Elizabeth on the southern coast, that has led to new concern that South Africa may be in for another period of racial violence.

Police arrested more than 300 blacks and battled with militant youths who burned more than a dozen government offices and shops in two black suburbs, Kabah and Kwanobuhle.

Authorities described the situation around Uitenhage tonight as "tense" and "uneasy" but said that it appeared to be under control. Police said, however, that further arrests are likely.

There were also islated incidents of stone-throwing at vehicles and police cars in several of South Africa's other troubled black townships on the last day of two-day commemorations marking the first anniversary of the death of the first blacks in racial demonstrations in Soweto. Police using tear gas dispersed the generally small crowds.

Fears were also renewed by an angry statement from the new president of the Soweto Student Representatives' Council, Tromfomo Sono, who pledged today that black youths "are going to fight our main enemy - Bantu [segregated black] education - as long as it frustrates our educational aspirations."

Although Soweto, the sprawling black ghetto outside Johannesburg, has witnessed only minor incidents - brief confrontations in which nine blacks were wounded by police gunfire last night - tension is still runnung high.

The pattern of the past week closely resembles last year's developments before the first outbreak, when sporadic stone-throwing in one area developed into serious confrontations with police and spread to black townships and tribal reserves throughout South Africa.

Uitenhage is an industrial city about 20 miles from Port Elizabeth, where more than 30 persons were killed in racial violence last fall. In Vitenhage, police said, the buildings destroyed included all six black schools, three government-run liquor stores, six shops, a funeral parlor, a community hall and a workshop.

Authorities said there were also widespread attacks on cars and delivery vans. Several vehicles were set on fire by crowds of youths, they said.

Most black schools in the country were closed today because of the two-day work and school boycott called by student groups to honor the 600 who have died since the violence began last June 16.

But employers reported an average 80 per cent attendance at major industries and businesses today, an increase from yesterday.

South African Minister of Justice and Police Jimmy T. Kruger praised police today for their "efficient preparation" and "good relationship with blacks," the two factors that had kept trouble to a minimum, he said.

Kruger also said that police used rubber bullets in Soweto yesterday for the first time and were "highly effective" in dispersing crowds.

This marks a major change in policy for South African police. Last year Kruger argued repeatedly that rubber bullets would not work because of African superstitions that would make blacks think they were immune from injury by any bullet.

Until the outbreak of violence at Uitenhage, police and government officials had expressed relief at the sense of serious conflicts, calling it important development for the country's domestic stability and its position abroad.