Black South African townships tense but calm after unrest ...

The riot-torn black townships south of Johannesburg appeared to be tense but relatively calm Tuesday - at least from the vantage point of police roadblocks. The South African police report that unrest this week claimed at least 26 lives, making it the bloodiest black disturbance since the Soweto riots of 1976. The rioting has occurred primarily in five townships in an industrial area about 30 miles south of Johannesburg.

The violence coincided with the introduction this week - amid great fanfare in Cape Town - of a new Constitution for South Africa. The Constitution angers blacks because it makes no provision for them politically. Another catalyst for the violence appears to have been a planned rent increase in the townships.

There has also been escalating black protest on other fronts. In recent weeks , for example, several black schools have had to shut down. Black pupils have demanded more say in running schools, an end to age restrictions the government places on who can enroll at each grade level, and an end to corporal punishment.

At the same time, South Africa's troubled economy - in its steepest recession since World War II - appears to be pushing blacks to the limit as they bear the brunt of rising unemployment and inflation. Protest over the rent increases, which apparently played a role in this week's unrest, is becoming more common.

There have been several incidents in which black policemen and black local government councilors were attacked by other blacks. Many blacks see such officials as effectively helping the government administer the system of apartheid.

Yesterday, as a police helicopter swept over the troubled areas, armed men in camouflage uniforms patroled the perimeters of some of the townships in armor-plated vehicles. It was impossible to get inside most of the townships.

One township that was open was Sharpeville, a small community with deep roots of black resistance. It was in Sharpeville in 1960 that police opened fire on blacks resisting South Africa's pass laws.

Yesterday, Sharpeville was strewn with debris. The entrance to the township had been cleared of the large boulders and tree stumps used earlier by blacks as barricades. But many of the interior streets remained littered with rocks and broken glass.

Some of Sharpeville's residents were going about their normal routine, walking across the field to the local supermarket. But there were also small crowds of blacks gathered at most street corners in the township. When a group of teen-agers was asked what had happened during the unrest, one told this correspondent circumspectly: ''A lot of bad things happened yesterday (Monday).'' One of the teen-agers shook his head angrily and walked away.

Aside from Sharpeville, this week's unrest hit the townships of Bophilong, Boipatong, Evaton, and Sebokeng - all south of Johannesburg and near the industrial city of Vereeniging. There have also been sporadic clashes with police in townships outside that immediate area, including Mamelodi, near Pretoria, and Thabang, a township near Welkom.

The South African police report that three black town councilors were ''murdered'' during the unrest in the townships near Vereeniging.

The police also report that 35 arrests were made in connection with the unrest. Some 55 buildings were damaged, including shops, churches, schools, homes, and the offices of the government development board which helps administer the townships.

Schools in all five townships were closed Tuesday. The police said late Tuesday they were remaining in ''full strength'' in the area but that the situation was ''under control.''