It has happened so infrequently in this country that the events remain in our national conscience, a scar reminding us forever of a terrible wound. The Boston Massacre, the Edmund Pettus Bridge in Selma, Kent State: armed men, acting under color of law and order, assaulting a crowd of unarmed civilians gathered to petition or protest. It is not tolerated, and when it happens, we do not forget.

Twenty-five years ago this week, South African police fired into a crowd of black demonstrators gathered at Sharpeville, a small town 70 miles south of Johannesburg. Using machine guns, they killed 69. On the very day of that anniversary this week, South African police fired into a crowd of blacks in the industrial city of Uitenhage. The government concedes that 18 are dead and another 29 wounded. Witnesses say the toll is much higher.

As is usual in these cases, the armed men claim to have been threatened and more or less forced to fire in self-defense. Blacks at Uitenhage tell another story. Thousands had gathered, they say, to board buses and cars taking them to the funeral of a black activist killed in a clash with police a few days before. The funeral services had been prohibited by the authorities, who feared trouble on the anniversary of the Sharpeville massacre. The police ordered everyone out of the vehicles and then, according to witnesses, opened fire with automatic rifles, pistols and shotguns at close range. One man in the crowd who rushed for shelter recalls the horror of bodies falling and the wounded lying on the ground, "moaning and writhing in agony."

It is hard for civilized people to imagine themselves firing an automatic weapon into a crowd of unarmed civilians. It is less troubling if the target is a faceless crowd en masse rather than a single individual who is staring hard into your eyes. It helps if you're with comrades who, by joining in, reinforce your self-righteousness. And it must be much easier if you are able to convince yourself that the people in your gun sight aren't quite as human as you or your grandmother or your children. That is happening with increasing frequency in South Africa -- 36 blacks have been killed by police in a single province in South Africa in the last 10 days. And though the government may not yet realize it, reaction is certain to grow, not only in foreign countries and black enclaves but also among the country's own white citizens for whom this nauseating violence brings home the unspeakable reality of apartheid. Apartheid cannot be preserved by machine guns. These blood baths only hasten the day of reckoning.