Riot police patrolled Soweto, the black township outside Johannesburg, today after two days of intermittent violence in which two blacks died.

Police used tear gas to break up several groups of youths milling about street corners and gathered in front of some schools. Police said that the youths were throwing stones and that two black men were injured when their car skidded out of control after being pelted.

Brig. Jan Visser, white police commander of the sprawling black township of a million people 10 miles southwest of Johannesburg, warned youths not to congregate. "I think we have played it low-key enough. If they want trouble, they're going to have trouble," he told reporters.

Yesterday, 176 blacks were arrested after an unprecedented march on police headquarters in white Johannesburg. A senior police official said officers used clubs to break up the protest against detention of 20 student leaders rounded up before June 16, the first anniversary of riots last year.

Tensions have been high in Soweto since June 16, when many blacks throughout South Africa mourned the victims of last year's rioting. The mood in Soweto before this week's violence was described by police as "ugly and confused."

Reporters traveling in Soweto today said there were incidents of anger against whites but the mood was less hostile than earlier. Two press cars were stoned, reports said.

Buses and trains ran normally and some schools reported increasing attendance as the day went on. Delivery-truck drivers, fearing that their vehicles would be looted and burned, stopped at the outskirts and shopowners had to pick up their bread, milk and vegetables.

[TEXT OMITTED FROM SOURCE] add two SOWETO - F - TTS - Boylan

In other developments in southern Arica:

Zambian President Kenneth Kaunda, hinting at a possible major intensification of the war in Rhodesia, told his countrymen to prepare around-the-clock defenses.

William Eteki Mboumoua, secretary general of the Organization of African Unity, meeting in the Libreville, Gabon, urged black African countries to set up their own combined defense force.