Sixteen black South African political and labor union leaders were charged with treason for making speeches in which they called for the unity and politicizing of the country's black majority and the unconditional release of political prisoners, it was revealed today.

The charges were spelled out in indictments at the start of the trial of the 16 in Natal Province.

The defendants are alleged to have committed treason by fomenting racial hostility, by trying to convince blacks that revolution is inevitable, labeling the white minority government as "the enemy" and by organizing meetings "at which revolutionary, inflammatory and bellicose songs were sung, slogans chanted and speeches made."

According to the indictment, these activities amount to treason -- which in South Africa is punishable by hanging -- because they were intended "to create a situation ripe for revolution."

The gist of the 580-page indictment is that the outlawed African National Congress and South African Communist Party, together with the South African Congress of Trade Unions, formed a "revolutionary alliance" that advocated such nonviolent programs in addition to its own violent activism "with the overall object of creating a climate favorable . . . for bringing about a revolution."

The scope of the indictment has startled South African lawyers. John Dugard, professor of law at Johannesburg's Witwatersrand University, said it was attempting to break new legal ground by making the spoken word, rather than any conspiracy to commit seditious violence, an act of treason.

"On this definition, some opposition parties in Parliament could also be charged with treason because they share some of the aims of the revolutionary alliance, such as giving people equal rights," Dugard said.

Stringent security measures were enforced as busloads of chanting black supporters arrived at the courthouse but were prevented from entering it. Ten were arrested for staging a demonstration outside.

The trial is expected to last about 18 months, during which the 15 men and one woman, all leading figures in the opposition to the white-minority government's system of apartheid, or racial separation, will be barred from political activity.

The accused spent several months in prison before being released on bail two weeks ago after a long court battle. But authorities, invoking a catchall security law, issued an order prohibiting the courts from granting bail to the prisoners before or during their trial.

Human rights organizations condemned the action as an attempt to immobilize the United Democratic Front by keeping its leading members, including copresidents Archie Gumede, 71, and Albertina Sisulu, 66, locked up for the better part of two years.

Also among the prisoners are several members of a black labor union, the South African Allied Workers Union, including its leader, Thozamile Gqweta.

Leaders of two organizations of Asian South Africans, the Natal Indian Congress and Transvaal Indian Congress, both founded by Mohandas K. (Mahatma) Gandhi during his early years in South Africa, are also among those charged.

In another development, The Associated Press reported that South African police said that they killed a black man today during an antiapartheid riot in Duduza, a township east of Johannesburg, and that a black mob in the same area pulled a white nurse, Gertina Aletta de Lange, from her car and stoned her until she fell unconscious.

De Lange, 51, was reported in critical condition.