PRETORIA, South Africa (AP) _ A judge ruled Thursday that some leaders of the nation's biggest anti- apartheid group committed treason by inciting violence during township unrest in 1984, a crime punishable by death.

The judge did not say which of the 16 defendants were guilty of treason. He acquitted three other defendants in the case, South Africa's longest-running political trial. It began in June 1985.

Supreme Court Justice Kees van Dijkhorst's decision was contained in a 1,521-page summary, which the judge began reading three days ago. Verdicts against individuals may not be announced until next week.

The three best-known defendants are senior leaders of the United Democratic Front who have been held 40 months without bail: Popo Molefe, 36, who was the coalition's national secretary; Terror Lekota, 40, who was its chief spokesman; and Moses Chikane, 40, a leader of its Transvaal Province branch.

Those acquitted Thursday were the Rev. Geoff Motselane, 41; Oupa Hlomuka, 35, and Patrick Baleka, 29.

The United Democratic Front, banned in February, was formed in 1983 to mobilize opposition to a new constitution that excluded the black majority from representation in Parliament.

All 19 defendants pleaded innocent to charges of treason, terrorism, murder and subversion related to allegations the Front and its affiliates incited violence when unrest broke out in black townships in 1984.

Van Dijkhorst said that because violence was an intended facet of the uprising in the townships, the state had proved the crime of treason.

The charges of murder, which also carry a possible death penalty, relate to the mob killings of black town councilors during the Vaal Triangle rioting.

Van Dijkhorst said one of the defendants, Thomas Manthatha, urged residents in the area to kill councilors. He also said the Vaal Civic Association, a Front affiliate to which several defendants belonged, encouraged confrontational protests that sparked rioting.

The Front grew into the largest opposition group in South Africa, encompassing more than 600 affiliated groups with about 2 million members. Most of its leaders are in detention, in hiding or barred from political activity.

Van Dijkhorst accepted the prosecutors' allegation that the group was supported and guided by the African National Congress, the main anti- government guerrilla movement. Front leaders maintained they endorsed the ANC's political goals, such as a one-person, one-vote system, but did not advocate the use of violence.

Van Dijkhorst has presided over the non-jury trial since its start and has sole responsibility for the verdict and sentencing.

There originally were 22 defendants in the trial, conducted for many months in the town of Delmas before being shifted to Pretoria. Three men were acquitted in 1986.