Treason trial opens opens today in South Africa. At issue: how much forthrightness will be permitted by officials

South Africa's biggest political trial since the marathon treason trial of 1956-61 starts today with the appearance in the Natal Supreme Court of 16 members of the United Democratic Front. The 16 face a main charge of treason and additional charges of terrorism and furthering the aims of the outlawed African National Congress and of communism. Treason and terrorism are capital offenses under South African law.

The trial is expected to last for months, perhaps even years. During that time the UDF, South Africa's most powerful extraparliamentary movement, will be deprived of some of its ablest leaders. The UDF, a legal group, opposes apartheid, South Africa's policy of strict racial separation.

The 16 accused include two UDF presidents, Archie Gumede, a septuagenarian who is the son of a former ANC president, and Albertina Sisulu, the wife of the imprisoned ANC leader, Walter Sisulu. Mrs. Sisulu is the only woman among the accused.

The core of the treason indictment is the allegation that all the accused associated themselves with, and supported the aims of, the ``Revolutionary Alliance.'' The state says this group was formed by the ANC and its allies, the South African Communist Party and the South African Congress of Trade Unions. Its ultimate objective, according to the state, is the ``armed seizure'' of power by the parties to the alliance.

But, while being committed to the use of violence to achieve its ends, the Revolutionary Alliance also espouses the use of ``nonviolent forms of political struggle'' in order to create a climate favorable to revolution, the indictment asserts.

Although the UDF is central to the trial, it is only one of five organizations indicted. The others are the Natal and Transvaal Indian congresses, the Release Mandela Committee (which supports the jailed ANC leader, Nelson Mandela), and the South African Allied Workers' Union. All predate the UDF, which was formed in August 1983, but all later became UDF affiliates.

The accused are alleged to have sought to advance the objectives of the Revolutionary Alliance through the convening of political rallies, the making of speeches at rallies and meetings, and the distribution of publications.

The essential focus of the trial will be on speeches allegedly made by the accused, rather than on overt actions. What is at stake is whether the speeches constitute ``verbal treason.''

It is unclear how the allegedly treasonable speeches differ from the generally forthright political oratory of opposition movements in South Africa.

``What could take them into the realm of treason is the purpose and intent with which they were made,'' one legal observer said. The question, he noted, is whether the state can establish a nexus between the speeches -- some of which predate the formation of the UDF -- and deliberate intent by the orators to further the aims of the Revolutionary Alliance.

The indictment includes a series of speeches made by the accused in which they are alleged to have advanced the cause of the Revolutionary Alliance or its objectives.

Mrs. Sisulu, to quote one speech cited in the indictment, is alleged to have told a meeting in Johannesburg last year that South Africa is ``ruled by greedy vultures, by inhumane people, by people who do not know the dignity of a human being.''

The state, which accuses Mrs. Sisulu of joining in the ``singing of revolutionary and inflammatory songs,'' interprets her speech as a bid to discredit and create hostility toward the government, with a view to mobilizing the masses against it.

The issue of when oratory becomes treasonable is of critical importance to opposition movements in South Africa because their freedom of action is so prescribed by security laws that verbal dissent is virtually the only avenue through which they can voice opposition.

The question the trial of the 16 may answer is: When is forthrightness permissible in South Africa, and when is it likely to be regarded as treasonable?