JOHANNESBURG, FEB. 24 -- In its harshest state-of-emergency crackdown on political opposition, the South African government today effectively banned 17 leading antiapartheid organizations, including the United Democratic Front (UDF) coalition, and prohibited the country's largest trade union federation from conducting political activities.

At the same time, the government announced it will release from detention some of the opposition groups' leaders, but under individual banning orders that include house arrest after working hours. Some UDF officials were individually banned from working for the organization, writing articles, giving speeches or granting interviews.

The ban was the most sweeping since groups opposing white-minority rule were outlawed in a clampdown in 1977 after the Soweto riots.

The emergency order, signed by President Pieter W. Botha, will limit the 800,000-member Congress of South African Trade Unions (COSATU) to labor activities, thereby effectively silencing one of the country's most potent voices of black majority opposition to the apartheid system of racial separation.

{In Washington, State Department spokesman Charles E. Redman called the measure a "giant step backward" and said that Assistant Secretary Chester A. Crocker called in South African Ambassador Piet G.J. Koornhof to "register our shock and distress at these inexplicable actions by his government," United Press International reported.}

The decree, announced by Law and Order Minister Adriaan Vlok, says that the UDF and the other restricted antiapartheid groups may continue to exist, and will not be prohibited from keeping financial records, performing "administrative functions" or "complying with an obligation imposed . . . by or under any law or court of law."

But they will have to obtain the law and order minister's permission to conduct any other activities, and those activities must not endanger public safety or law and order.

Some of the affected organizations said they will challenge the restrictions in court.

Vlok said he was convinced the new regulations will "contribute to a climate of stability, peaceful coexistence and good neighborliness among all population groups."

However, antiapartheid leaders predicted that the decree, which does not require parliamentary approval, will trigger a renewal of the kind of violence that led to the imposition of a nationwide state of emergency 20 months ago.

The Rev. Alan Boesak, a founder of the UDF, said, "Every single peaceful action we can take has now been criminalized." Boesak said the government's action will drive more antiapartheid activists underground and leave them with violence as the only available option.

Boesak and other antiapartheid leaders suggested that the government's motive for implementing the curbs now was to head off a black boycott of nationwide municipal elections scheduled for October. The UDF has opposed black participation in the planned township elections.

Antiapartheid activists also noted that the new restrictions came just a week before two parliamentary by-elections in which the far-right Conservative Party is favored to defeat the ruling National Party.

Anglican Archbishop Desmond Tutu, the 1984 Nobel Peace laureate, called the decree an "unmitigated disaster," and said that the government's idea of reform is "to smash all possible political opposition in the country, no matter how peaceful or lawful, and to rule with the jackboot."

Tutu, in a news conference, called on western nations to respond to the new restrictions by stepping up sanctions against Pretoria.

{In London, Foreign Secretary Geoffrey Howe said the British government, which has resisted Commonwealth nations' pressure for sanctions, is "totally opposed to repressive measures of this kind," Reuter reported.}

Vlok said that despite emergency regulations already in effect, some opposition groups had "persisted in establishing, maintaining and promoting a revolutionary climate." The new regulations, he said, would make it possible to restrict the activities of those groups.

In November, Security Police Chief Johann van der Merwe signaled the possibility of such a crackdown when he told reporters that "legal radical organizations" such as the UDF and COSATU are of more concern to the police than illegal groups such as the African National Congress.

Today, van der Merwe, in a statement, quoted ANC President Oliver Tambo as calling for the creation of "mass democratic organizations" as part of what the police chief called "the revolutionary onslaught." Tambo, van der Merwe said, specifically named the UDF and COSATU as examples.

COSATU, which had begun to fill the organizational vacuum created by the detention of most of the UDF's national and regional leaders, is now prohibited from campaigning for the legalization of outlawed organizations like the ANC; from urging the release of detainees; from promoting boycotts of local black elections, and from calling for disinvestment or sanctions against South Africa.

It also is prohibited from "stirring up" opposition to proposed negotiations between moderate black leaders and the government over constitutional reform.

Since its formation in 1985, COSATU has become increasingly politicized and it recently emerged as one of the main antiapartheid forces in South Africa, in addition to becoming the focal point of the black labor movement.

At a national convention here last year, its president, Elijah Barayi, declared, "This intransigent government will not hand over power. The black majority will have to seize power from the intransigent government."

Last May, two powerful bombs exploded in the basement of COSATU's Johannesburg headquarters, causing structural damage so severe that the federation has been unable to use the building. The bombing remains unsolved.

The list of organizations effectively banned also includes the Detainees' Parents Support Committee, the Azanian People's Organization, the Release Mandela Committee, the South African Youth Congress, the National Education Crisis Committee, the Soweto Civic Association, the Soweto Youth Congress and the South African National Students Congress.

Among those served with orders individually restricting their political activities were both copresidents of the UDF -- Durban lawyer Archie Gumede and Albertina Sisulu, wife of imprisoned ANC leader Walter Sisulu.