DIPLOMACY : Keeping the Peace in S. Africa : U.N. monitors placed themselves between angry blacks and determined police. The monitors’ presence defused the tension.

On one side were almost 500 angry black youths, seething over the police shooting of a man the day before and now determined to march on the local police station.

Facing them was a jut-jawed white police colonel, who had a letter from his commander forbidding the march and the firepower to back it up.

In the middle was a plump, bespectacled man in a pin-striped suit--Hisham Omayed of the United Nations.

“We will talk to the police,” Omayed told the crowd. “But you must make sure this march is peaceful. Don’t let us down.”

After heated negotiations, the police colonel backed down. The march proceeded peacefully through this tense township east of Johannesburg and the United Nations’ 10-member monitoring team, the first foreign mission of its kind ever allowed in South Africa, chalked up its first victory.

The team had come to South Africa with a fairly tame mission: to observe the week of strikes, marches, rallies and sit-ins launched by the African National Congress in South Africa and report back to the United Nations. But to nearly everyone’s surprise, the U.N. observers intervened to head off at least half a dozen potentially bloody flare-ups and, by their mere presence, likely defused many more.

“These guys are going to help the police behave,” said Moses Yabo, an ANC leader in Daveyton. “I wish they’d been here yesterday. The police behaved like dogs then.”

Few in South Africa want a full U.N. peacekeeping force to stop the violence or even a U.N. mediator for the now-suspended constitutional talks. President Frederik W. de Klerk says those are problems that South Africans must face themselves, and the ANC agrees. But there is a growing consensus in South Africa that a modest band of U.N. monitors, with limited authority, could help avert bloodshed, as they proved this week.

And Cyrus R. Vance, who spent 10 days in the country as the U.N. secretary general’s special envoy recently, is expected to make just such a recommendation soon to the Security Council.

Time and again this week, the U.N. monitors found themselves at the scene of looming confrontations that cried out for an impartial mediator. Omayed, the team leader from Ghana, had promised that his efforts would be “more than a public relations exercise.” Helicopters and cars were put at the team’s disposal.

ANC chapters across the country had planned hundreds of rallies and marches. Some had been approved by the government. But others, particularly in areas where support for the government’s right-wing opponents is strong, had denied the ANC permission to protest.

In those cases, the U.N. monitors frequently became the broker between the desire of ANC supporters to peacefully exercise their right to protest and the desire of police--and right-wing whites--to prevent marches that they thought would turn violent.

In Daveyton, the residents were angry because a policeman had killed an elderly black man the day before. The police said the man had pulled a spear on them. But ANC activists said the man was unarmed and had been shot through the windshield of his truck. The activists wanted to march in protest to the police station. The area police commander had denied them permission. When his letter was read to the crowd, Omayed realized serious trouble was brewing.

“That letter wasn’t very diplomatic or very tactful,” Omayed said later, after he and his escort intervened, heading off a confrontation and allowing the residents to march.