South Africa's segregated public schools, a major cause underlying the political violence of the past year, were the scene of new clashes today in two of the country's major population centers.

Several thousand students, parents and teachers converged on nearly two dozen schools and confronted police in the mixed-race townships outside Cape Town in an unsuccessful attempt to reopen them. The protesters were defying a government order that closed the schools 11 days ago because of unrest in the western Cape area.

The most serious confrontation took place at the Alexander Sinton senior secondary school in Athlone, a "colored," or mixed-race, township, where police fired rubber bullets and tear gas at a crowd of 4,000 persons and arrested 173 adults and students.

In Soweto, the country's largest black urban center, students rampaged at two black high schools. Police fired birdshot, rubber bullets and tear gas to break up a crowd of students at a school that authorities had ordered closed yesterday.

The Cape Town schools were among more than 460 colored elementary and secondary schools and teachers' colleges ordered closed Sept. 6 by Carter Ebrahim, the minister for colored education, after 10 days of political unrest in the area. The closing, which has been widely criticized by parents and educators, affected nearly 500,000 students.

Ebrahim said he ordered the shutdown because the schools had become centers for students organizing protests and boycotts. But yesterday, in an apparent effort to defuse confrontations, he said the schools would likely reopen Oct 1.

The clash at Alexander Sinton reportedly began early today when a contingent of police began arresting members of a crowd of at least 400 students, parents and teachers. The police were trapped inside the building and fired tear gas into the grounds at the gathering crowd.

Meanwhile, witnesses said students outside the grounds seized buses, delivery vehicles and garbage trucks and blocked off a six-block area around the school, preventing police from coming to the rescue of their colleagues.

Armored police vehicles then surrounded the area, and police fired rubber bullets and tear gas at a nearby parking lot where another crowd had gathered. The school principal, identified as C. Desai, who had been arrested early in the morning, was released from a police van to talk to leaders of the crowd.

Eventually the crowd dispersed and the police escaped. Arrests included 107 students and 66 adults, police said. They were charged with trespassing, and by tonight many had paid $15 fines and been released, according to Maj. Steve van Rooyen, a police spokesman.

At Livingstone senior secondary school, Principal Robert Samuel Evans said he defied orders from six policemen on the grounds and opened the school this morning. More policemen arrived, and a crowd Evans said had grown to nearly 1,000 students and their parents dispersed quietly.

Police also fired tear gas to disperse a crowd near Belgravia High School in the township. An undisclosed number of persons were injured, including a high school student with birdshot wounds in his head and back and a trolley driver knocked unconscious by a rock thrown through his windshield.

There were scattered incidents at a half dozen other schools and unconfirmed reports tonight that four teachers had been arrested under the state's sweeping internal security act.

The United Democratic Front, a leading antiapartheid activist coalition, said the police action today would not "deter people in their determination to reopen the schools." A front spokesman denounced the "heavy-handed actions" of Ebrahim and of the police today in keeping the schools closed.

The 1,000-member Concerned Teachers Coordinating Committee yesterday condemned the "senseless" closings as an "extreme and unwarranted punitive measure against the entire community." It called for the government to reopen the schools, release people who had been detained, and remove the Army from the townships. It also called for an end to racially segregated education.

Naledi High School in Soweto had been shut down yesterday under emergency regulations after students allegedly stoned school buildings. They accused the school's principal of calling in the police after an incident in which the students tried to attack the house of a man they said had fatally stabbed a schoolmate.

Today's trouble reportedly began between students who wanted to boycott classes and others who wanted to attend. The principal allegedly was manhandled by the protesters, who later stoned vehicles.

The principal at Hlengiwe High School, where 746 students were arrested last week, also was manhandled today by students who accused him of conspiring with the police, authorities said. Classes were suspended again today.