The Supreme Court on Friday convicted four people of treason, three of them officials of the anti-apartheid United Democratic Front, after the longest trial in South Africa's history.

Popo Molefe, Terror Lekota and Moss Chikane were senior members of the UDF, South Africa's largest anti-apartheid organization. The treason charge carries a possible death sentence.The case, which has lasted 37 months and involved 278 witnesses, is one of the most important since the treason trial that sentenced nationalist leader Nelson Mandela in 1964 for plotting to overthrow white rule.

Judge K. Van Dijkhorst found that the UDF leadership had adopted in 1984 and 1985 a policy of mass protest by violent means aimed at making South Africa ungovernable.

After he was sentenced, Chikane, the Transvaal provincial secretary of the UDF, told reporters: "The UDF is a non-violent organization."

The fourth man convicted of treason was a local anti-apartheid activist, Thomas Manthata, who had links with both the UDF and a rival group that stressed black self-reliance in opposing the government.

Manthata was also found guilty of terrorism along with seven other activists who had organized rent boycotts during the 1984-86 nationwide black uprising.

Van Dijkhorst found six of the original 19 accused not guilty and they were discharged.

Earlier in the trial, six other accused had been discharged.

Lekota, UDF publicity secretary and wearing a big UDF badge, said: "We have nothing to be ashamed of. The only people to be ashamed are the people who rule this country."

Archbishop Desmond Tutu was in the public gallery for Friday's hearing, as were a number of foreign diplomats including U.S. Ambassador Edward Perkins.

"Every time we try to oppose the state they criminalize us," Tutu told reporters.

The case is seen as a trial of the 2 million-member UDF as much as of the defendants.

During his judgment, spread over four days, Van Dijkhorst found the organization had behaved treasonably by stirring up violence in the Vaal Triangle area south of Johannesburg during a black uprising in 1984.

The cream-painted courtroom at Pretoria's Palace of Justice was packed with relatives and supporters of the accused, who included some of the most famous names of the South African opposition.

Tutu sat through the proceedings, but left early after embracing several people in the silent audience.