Thousands of blacks demonstrated throughout South Africa today to mark the anniversaries of two landmarks in the history of black resistance to apartheid and honor the dozens of protesters killed by white police during the incidents.

It was a day of strong emotions but generally little violence as demonstrators in four major urban centers commemorated the 1960 killing of 69 protesters at Sharpeville and last year's killing of at least 20 outside Uitenhage. The main exception was the city of Durban, where the downtown area erupted in chaos briefly this morning when police used whips and tear gas against hundreds of demonstrating children and students.

Today's relatively peaceful demonstrations followed some of the heaviest violence in recent weeks with police reporting that 13 blacks were killed last night, all in incidents involving other blacks. The worst toll was in Guguletu, a black township outside this city, where seven men were found hacked to death.

The Sharpeville massacre long has been etched in black memories as symbolizing the potential ruthlessness of white-minority rule. But the date's significance was compounded by last year's shootings in the township of Langa, which have come to symbolize the past 19 months of unrest in which nearly 1,200 persons have died, two-thirds of them from police action.

The Langa incident took place on the same date as Sharpeville by coincidence. Thousands of mourners were on their way to neighboring Kwanobuhle township for an unrest victim's funeral that had been postponed and rescheduled by police. Police, who claimed that the crowd threw rocks and bottles at them, opened fire and within minutes had cut down at least 20.

Witnesses said that there had been no provocation and that many of the victims were shot in the back. A subsequent inquiry by a local judge largely rejected the police version of events but did not call for disciplinary action.

Today's largest and most emotional protests took place in Uitenhage and neighboring Port Elizabeth. Thousands honored a day-long strike, leaving central shopping areas almost completely deserted.

More than 30,000 mourners gathered at the Kwanobuhle stadium and later walked to an adjacent cemetery where a large marble tombstone was erected honoring the dead. On its face was engraved a map of Africa with a warrior carrying a shield in his left hand and an upraised spear in his right.

"We've seen blood on our streets," said Dutch Reformed Rev. Allan Boesak, a founder of the opposition United Democratic Front and the main speaker. "When I was here last the blood was still wet. We'll never forget, because we can never forget."

Boesak ridiculed the government's efforts at cautious political reforms, and told the crowd, "We can't afford to step back one inch. The struggle must continue every day . . . 1985 was a difficult year for us, and 1986 will be a more difficult year."

Township residents have maintained that 43 were killed at Langa.

Winnie Mandela, wife of the imprisoned black nationalist leader, Nelson Mandela, said in a written message read today that the victims had "fallen in battle." She said she knew that many more blacks than whites would die in the freedom struggle, but "what we do not accept, and never will, is that blood will continue to flow on our side only." She asked, "Why must the price of apartheid be so high on our side?"

Police kept an unusually low profile in Uitenhage and in Johannesburg, where police vehicles and an alleged informer were attacked with rocks by protesters from the University of Witwatersrand.

In Durban, however, violence flared when hundreds of school-age children gathered for a protest. Police cordoned off two main streets and chased children into shops, hauling them out and arresting them. Witnesses said tear gas was fired into crowds of shoppers, many of whom panicked, dropping their parcels and running from the fumes.

In contrast with the Langa deaths, last night's victims all allegedly were killed by other blacks -- violence that has claimed about one-third of South Africa's unrest victims. The seven men killed in Guguletu were apparently the latest victims of a power struggle between a local black leader and supporters of the United Democratic Front.

The violence began two nights ago with the killing and mutilation of two men allied with the local leader, Johnson Nxobongwana. His backers call themselves "the Fathers," while rival front supporters are known as "the Comrades," and it was not immediately clear to which side the latest victims belonged.

Two policemen and a bus passenger were shot dead during an ambush of a bus in the black "homeland" of KwaZulu outside Durban, according to Zulu Chief Gatsha Buthelezi. A man and a girl were burned to death when a mob torched 13 houses in a township near East London, and a Soweto woman died from a burning tire placed around her neck -- the method used by some militants to kill blacks considered "collaborators" with the apartheid system.

UDF national chairman Curnick Ndlovu, noting the nature of last night's deaths, told the Kwanobuhle rally, "The apartheid system is trying to keep us divided and quarrelsome so that we end up fighting each other and not the Pretoria regime -- and it is having too much success at this turning of brother against brother."