JOHANNESBURG, DEC. 8 -- South African security police today detained a prominent antiapartheid activist, Eric Molobi, amid rising fears by black nationalists that a crackdown on government opponents is imminent, his attorney said today.

Molobi, one of the few national executive committee members of the United Democratic Front coalition who was not under detention, was arrested as he left the downtown Johannesburg office of attorney Priscilla Jana after discussing the creation of a trust fund for recently released African National Congress Chairman Govan Mbeki, Jana said.

Of the original 19 United Democratic Front national leaders named in April 1985, only three are not in detention, in exile or underground.

Jana said police confirmed to her that Molobi, who is also national coordinator of the National Education Coordinating Committee, was being held under emergency regulations and had not been charged. The police command said its policy is not to confirm detentions under the emergency laws.

The United Democratic Front said in a statement that it was "shocked and angered" by Molobi's detention, which it said "must convince all reasonable people that the prospects of a peaceful, negotiated transition to democracy have virtually disappeared."

UDF sources said they feared a new general roundup of officials of the organization, which consists of more than 700 groups opposed to the system of strict racial segregation known as apartheid. UDF officials have estimated that more than half the 1,800 South Africans confirmed by monitoring groups as currently being under detention are members.

Senior government officials hinted broadly at new police actions against "lawful radical organizations" that allegedly use legal loopholes to fan a revolutionary climate by encouraging popular rejection of black town governments.

Warning that new attempts were being made to organize boycotts and strikes, Security Police Chief Lt. Gen. Jan Van der Merwe said at a briefing of South African parliamentary reporters last week that existing security legislation was not adequate for dealing with the threat and that emergency powers would have to be used more extensively to deal with "radicals."

In anticipation of an annual "Christmas Against the Emergency" campaign being organized by the UDF to protest South Africa's state of emergency, security police last week raided the offices of several antiapartheid groups in Johannesburg, seizing pamphlets and other material.

Organizers of the campaign said they had not urged strikes or consumer boycotts, and had merely asked residents of black townships to "act with dignity" during the campaign, light candles at specified times and not drink alcohol in public.

Fears of a new clampdown on dissent also coincided with increasing signals by senior government officials that Mbeki's release from prison on Nov. 6 might not, as was widely expected, lead to the release of his codefendant, ANC leader Nelson Mandela, and other security prisoners.

Mbeki served 23 years of a life sentence for sabotage and treason.

Law and Order Minister Adriaan Vlok has described Mbeki's conduct since his release as disappointing, and Van der Merwe has said the 77-year-old black nationalist was being exploited by the outlawed ANC to promote the guerrilla group's image in South Africa.

Previously, senior government officials had said that reaction among blacks to Mbeki's release would determine whether Mandela and other security prisoners are released.

The motive behind their strategy was believed to be to press credible black leaders such as Zulu Chief Mangosuthu Gatsha Buthelezi to negotiate a power-sharing agreement between South Africa's 26 million blacks and the Pretoria government, which is elected by the country's 4.5 million whites. Buthelezi and most other black leaders have said they cannot join negotiations while Mandela is imprisoned.

Antiapartheid activists said they interpret the government's recent hard-line statements, following six months of seemingly conciliatory moves, as a reaction to the Conservative Party's protest over Mbeki's release.