The troubles of Africa's only two white-minority governments took major turns today, as South Africa's militant blacks launched new demonstrations here and the leaders of South Africa and Rhodesian held a surprise summit.

The demonstration broke out in Soweto, Johannesburg's sprawling black township, when an estimated 6,000 students attempted to disrupt exams by burning textbooks at five high schools. They were protesting the African education system, the continued in last year's riots and other long-detention of black students involved standing grievances.

Riot police dispersed the mobs of miltant youths with teargas in the first major protest demonstration this year - and the first step in a new anti-government campaign, according to Soweto students.

In Cape Town, Rhodesdian Prime Minister Ian Smith arrived this morning for unannounced talks with Prime Minister John Vorster on the Rhodesian crisis - their first meeting in five months and the first since the collapse of the Anglo-American settlement effort.

No statement was made on the specifics of their discussions, but the meeting took place as Rhodesdia reaches a critical period politically, militarily and economically.

It is thought that Smith appealed for new aid for his beleaguered government, in light of Vorster's speech 10 days ago promising not to push Rhodesia into acceptance of the Anglo-American peace plan.

After the three-hour meeting, Vorster said, "Mr. Smith informed me of his government's determination to explore further avenues for settlement."

Vorster, the only politician with the leverage to force Rhodesian acceptance of an internationally backed settlement plan, added: "Further talks are envisaged."

There was serious speculation that Smith requested the meeting to test South African support by requesting aid in containing the rapidly escalating guerrilla war and for Rhodesia's gravely troubled economy.

Vorster also faces new troubles at home. The outbreak of violence today follows a week of campaigning by militant youths, who have been ruging Soweto students to boycott, the exams that began Monday and run for three weeks.

About 4,000 students chanting "freedom" songs and waving clenched fists marched on Meadowland high school, the main trouble spot, according to police. After dumping books they had brought from other schools on the school lawn, they set the piles alight and urged students inside the building not to take exams.

One black student said all the books burned today were in Afrikaans, the controversial language of South Africa's early Dutch settlers that sparked the first series of riots in Soweto last June.

After the first disorders, students called on the government to install a universal education system instead of different standards for the country's four race categories: blacks, whites, Asians and "Coloreds" (mixed race).

The government has since repealed the requirement that blacks be taught in Afrikaans and in January it promised to provide free textbooks, open adult education centers and provide new opportunities for black teachers to get advanced degrees.

These steps have only further angered students as merely token gestures. The situation has been simmering since last week, when several schools suspended exams after students tore up their test papers.

Several black principals and white officials have appealed to students not to join the exam boycott. Jaap Strydom, regional director of Bantu (black) education, appealed to parents yesterday over the local black radio station to dissuade their children from taking part in the demonstration.

There have been several reports of intimidation by militant students.

In response to the incidents today, Soweto's 250 schools closed and police imposed an 8 a.m.-to 3 p.m. curfiew on students, promising "tough action" against those not at school.

Brig. Jan Visser, Soweto police chief, told teachers to warn pupils that if trouble continued the police might be forced to shoot.

There was no shooting, no serious injuries and only one reported arrest today leading one black journalist to praise police for their restraint.

(In Pretoria, United Press International reported, the Roman Catholic Church reaffirmed its commitment to inegrate its schools despite government threats to cut off funding.)