Four black trade union officials who were the last defendants in one of South Africa's largest political show trials today were acquitted of treason after the judge threw out a key part of the state's case.

Meanwhile, the government ordered the expulsion of Newsweek correspondent Richard Manning, the second foreign journalist to be marked for deportation since the nationwide state of emergency was imposed 11 days ago. The order came amid indications that Pretoria would maintain the emergency even after tough new security legislation becomes law later this week.

The trial in Pietermaritzburg, which began 13 months ago, was one of two in which the government had sought to hamstring the United Democratic Front, the militant antiapartheid coalition here, by charging more than three dozen of its leaders and their trade union allies with treason.

It ended abruptly when the prosecution conceded its case had collapsed. The move followed a landmark ruling two weeks ago that most of the state's audio and video tapes of allegedly treasonable activities by the defendants were not admissible.

Presiding Justice John Milne said the state had failed to establish that the tapes, some of which had suspicious gaps, had not been tampered with, and that they accurately represented what the prosecution claimed they were -- records of political meetings both within and outside South Africa at which the defendants spoke or appeared. The judge said the state also failed to establish that the voices heard on the tapes actually were those of the defendants.

After the ruling, the prosecution asked for an adjournment. When court reconvened this morning, Michael Imber, attorney general for Natal Province, told the judge that the state had decided to stop the prosecution.

Milne then acquitted the four defendants, all of them top officials of the South Africa Allied Workers' Union, considered one of the country's toughest new trade unions. It is a general workers' union and is especially strong in the troubled East London area, where blacks are militant and politically well-organized.

The four officials first were arrested nearly two years ago and held nearly eight months before being released on bail that included stringent restrictions that banned them from any political activities.

The four were carried on the shoulders of supporters outside the courthouse where they were able to speak freely for the first time since their arrest.

"I feel like I've been in a political deepfreeze for two years," said Richard Gqweta, the union's national president, who said he and the others remained committed to "continue the work."

Twelve codefendants who are top officials of the United Democratic Front were freed last December when another portion of the state's case collapsed. Another 22 UDF leaders, who were first arrested nearly two years ago, remain on trial in a similar treason case in a town east of Johannesburg. They remain in prison during the trial, which began in January.

Meanwhile, the ordered deportation of Newsweek's Manning followed the magazine's pointed cover story last week on South Africa, in which President Pieter W. Botha was accused of turning "a racist regime into a police state" and of giving police "a license to kill."

Home Affairs Minister Stoffel Botha, in issuing the order, said only that he considered it "in the public interest to order your removal from the Republic of South Africa." Manning said Newsweek would appeal the action, which requires him to leave South Africa by midnight Thursday.

Last week's edition, whose contents flaunted the emergency's stringent restrictions on news coverage here, did not appear in South Africa because Newsweek's distributors feared the government would hold them liable.

Manning, 35, who is an American citizen, came to South Africa last September following the expulsion of another Newsweek correspondent, Ray Wilkinson, who is based in Nairobi.

The expulsion order follows a series of strong warnings from officials that they would not tolerate the reporting of "subversive statements" or other "violations" of the sweeping emergency restrictions. Home Affairs Minister Botha two weeks ago ordered the expulsion of CBS cameraman Wim de Vos, a Dutch citizen, who was deported Tuesday night after his appeal to the minister and later to a Johannesburg court failed.

The government today appeared to be weighing whether to retain the state of emergency after President Botha signs new legislation allowing police to detain political activists for up to six months without charge and to declare and limit access to designated "unrest areas."

Some opposition politicians and political analysts had predicted that Botha would lift the emergency once the new measures were law. But Deputy Information Minister Louis Nel told the Johannesburg Star that the government still had not considered ending the emergency.

The Star said other sources indicated it would only be lifted when officials were certain they would not have to reimpose it. An estimated 3,000 persons have been detained under the measure.

The progovernment Citizen newspaper quoted sources as saying that the emergency had been "so successful" in stabilizing the unrest situation in South Africa that officials planned to maintain it while keeping the new measures "held in reserve."

The government has reported that 57 people have died in unrest since the emergency was imposed. Last week's death toll was one of the highest in recent months, although officials have maintained that the total number of incidents is dropping.

The latest casualties, the government reported today, were two blacks who were burned to death by other blacks in separate incidents. Officials said two other blacks were rescued by security forces from being killed in a similar manner. Because of state restrictions on press coverage, it was not possible to confirm the report independently.

Police served summonses on two religious leaders accused of sheltering black refugees from the violence-wracked Crossroads squatter camp outside Cape Town.

The two leaders, a white Anglican cleric and an assistant to a Moslem clergyman, were charged with breaking a law prohibiting blacks from working and living in white areas. The two are harboring about 100 women and children who fled from their homes two weeks ago after armed vigilantes clashed with militants in the area.