South Africa's biggest political trial in 21 years began today with 16 black and Asian civil rights leaders facing a charge of high treason.

The trial in the Natal provincial Supreme Court opened with defense lawyers asking that the 580-page indictment be thrown out because they say it is vague and unspecific.

Describing it as an attempt to establish the crime of "verbal treason," Ismael Mohamed, who heads the defense, said the indictment's terms were so broad they could apply to the whole parliamentary opposition, particularly its best known liberal member, Helen Suzman.

If the defense strategy succeeds, the case could collapse within a week, lawyers said. If it fails, a two-year marathon will get under way that, together with a similar trial in Transvaal Province of 22 other leaders of the country's main black civil rights organization, the United Democratic Front, will tie up the entire top leadership of the front and effectively prevent them from playing any political role. The trial of the 22 is due to start in October.

It is widely believed in political circles that the authorities brought the treason charges to put the democratic front out of action, believing it to be behind the racial unrest that has wracked South Africa for the past 11 months.

The maneuver failed. Although the 22 prisoners in Transvaal have been held in custody for four months and the 16 in the trial here have been given bail on the condition that they take no part in politics, the unrest in the black townships has continued to intensify.

Now the government has declared a state of emergency and is trying to stamp out the unrest by arresting the leaders of hundreds of grass-roots organizations, most of which are affiliated with the democratic front.

The massive indictment in the Pietermaritzburg trial cites hundreds of political actions by the 16 leaders between 1980 and 1985, including speeches criticizing the apartheid system of segregation and calling for the release of political prisoners, the distribution of pamphlets and the singing of freedom songs, which it claims amounted to a conspiracy to create a revolutionary atmosphere with a view to overthrowing white-minority rule by violence.

The indictment is based on the contention that the United Democratic Front, together with the outlawed African National Congress, South African Congress of Trade Unions and South African Congress Party, formed a "revolutionary alliance" that advocated nonviolent programs to further its violent aims.

The scope of the indictment, and its attempt to make the spoken word an act of treason -- which is punishable by hanging -- has startled lawyers.

Mohamed referred to the crime the defendants are charged with disparagingly as "talk-talk treason" in his address to the court today as he argued for dismissal of the indictment.

The prosecutor, N.C. Gey van Pittius, is scheduled to reply to the defense arguments Tuesday.

Mohamed noted that the distribution of pamphlets calling for the release of the imprisoned African National Congress leader, Nelson Mandela, was cited as one of the actions forming part of the alleged conspiracy and said: "On this basis the entire official opposition in Parliament, and especially Helen Suzman, could be charged with treason."

"In a country as deeply divided as this, it is inevitable that there will be a furious and passionate debate about the nature of society," Mohamed said. "That is part of our national heritage. The authorities must be subject to criticism, and severe criticism."

The last big political trial was when Mandela and other leaders of the African National Congress were sentenced in June 1964 to life imprisonment for trying to overthrow white rule. The last major treason trial was in 1955, when 166 black leaders were charged but acquitted four years later.

Mohamed also argued that the indictment was defective in charg-ing the prisoners collectively on the basis of "different acts committed in different places at different times."

He asked Judge John Milne to order that the joint indictment be withdrawn and the 16 men charged again separately. When Judge Milne asked how this would benefit the prisoners, Mohamed replied: "I think, my lord, that if that were to happen, it might be the last that we see of this case" -- an implicit suggestion that the administration of President Pieter W. Botha regarded it as a political failure that it would drop if given the chance.

The trial has aroused strong feelings in the black community, and armed police with dogs patrolled the streets near the red-brick courthouse, which is behind a high security fence, to prevent demonstrations. A few hundred black people gathered on the pavement opposite the courthouse throughout the day, but there were no incidents.

Despite the gravity of the case and the racial tension it has aroused, there was a calm, almost genteel atmosphere in the court itself, which was redolent of the province's British colonial past.

Natal is the province of the English minority, and the proceedings had about them a touch of British colonial rule in India as the flamboyant Indian lawyer pleaded his case before the politely attentive judge with an accent that had a ring of the English upper class.

A visiting U.S. federal appeals court judge, Nathaniel Jones of Cincinnati, found it all very surprising. "I guess the problem here is not one of procedure but of the underlying body of law," he observed during a tea break.