A South African judge convicted 11 black students of sedition today for organizing demonstrations during the Soweto riots, using a broad definition of sedition that legal observers said includes many antigovernment street protests.

If the sentencing expected later this week is harsh, the "Soweto II" verdict could add another ominous legal restraint to political protest in this country, where civil rights, especially those of blacks, already are statutory penalties for sedition.

Friends and colleagues of the defendents-10 men and one woman ranging in age from 18 to 23-packed the small suburban courtroom here to hear the judgment despite the fact that it is 35 miles from the vast black ghetto outside Johannesburg that is home for mor than a million blacks.

The case against the former high school students is one of the most important political trials in recent years because it is regarded as the government's attempt to explain the reasons for the 16-month upheavel among blacks that jolted the white minority government's confidence and left more than 700 people dead.

Through this trial the government hopes to demonstrate to its own white constituents and overseas investors and critics that the unrest was the consequence of agitators who stirred up school children for political purposes. Most blacks contest this official version of the root causes of the turmoil of 1976 and 1977, describing it instead as a spontaneous outpouring of pent-up fury against a system that keeps blacks as second-class citizens.

A government-ordered probe into the background of the protest on June 16, 1976, that began the unrest has yet to make its findings public. Many government critics suspect it found that poverty, inferior schooling and housing and race laws fueled the unrest. These critics speculate that the probe's report has been delayed to avoid prejudicing the state's case in this trial.

Among the student leaders found guilty today was Daniel Sechaba Montsitsi, 23, a former president of the Soweto Students' Representative Council, whic was formed to air student grievances during the unrest. He is one of several students who have charged police with assault during their confinement.

Police have denied the charge, but a trial is expected on the matter.

The "Soweto 11" are only a handful of the thousands who went into the streets in 1976 and 1977 to voice anger at the political and economic inequalities of apartheid. Most of their fellow leaders have fled the country to avoid arrest. Two exiled student leaders, Tsistsi Mashinini and Khotso Seathlolo, are listed as conconspirators with the "Soweto 11".

Because of the sysmpathy of the rest of the community with the students, the council attained an inordinate, though short-lived, influence over events in Soweto. The government banned it in October 1977.

Transvaal Province Supreme Court Judge Hendrik Van Dyk said today he believes the state proved the council organized the student protests in 1976 and 1977. He found these demonstrations seditious since they were intended to "subvert, defy or assail" the authority of the state, the police or the Education Department.

"The students knew that to organize a gathering in that atmosphere would provoke a confrontation with the police and therefore it is seditious," the judge said at one point.

Van Dyk added that he did not regard violence as an essential element in a seditious act.

Among the incidents organized by the student leaders that Van Dyk regarded as sedition were burning of school textbooks; a demonstration protesting the visit of then-secretary of state Henry Kissinger in September 1976; a campaign to get adults to stay away from work to protest apartheld; and the initial student protest of June 16, 1976.

Van Dyk dismissed the argument of defense counsel Ernest Wentzel that the student council was a peaceful organization that had no other way of voicing dissatisfaction with an inferior school system.

Van Dyk's judgment did not attempt to address the merit of the student grievances that led to the demonstrations, nor did it define the police role and responsibility in confrontations with the protesting students. Earlier police testimony at the trial established that on June 16 the police had no instruments of crowd control except guns and that even though they had an opportunity to retreat while he students cooled off, they did not do so in order "not to lose face."

Montsitsi and his condefendants were kept in solitary confinement from seven to 12 months before they were charged last July. They have remained in jail during the trial, during which the state brought 106 witnesses. Many were black and several testified in closed session at the prosecution's request.