S. Africa tightens the screws. Pressure from right and recent bombings prompt tough action against leading anti-apartheid group and Mozambican laborers

Days after the United States imposed sanctions on South Africa, the government here has tightened its clampdown on groups militating for an end to white political domination. Besides producing an additional squeeze on the opposition, this week's moves by the government would seem to further undercut charges from the extreme right that Pretoria has been weak in responding to the political unrest of the past two years.The new show of muscle is in line with the goals of the four-month nationwide state of emergency.

In back-to-back moves Wednesday and Thursday, the government pressed its offensive against the country's two main anti-apartheid organizations. The two groups are: the outlawed African National Congress, which is seeking the armed overthrow of the government; and the United Democratic Front, a confederation of some 600 anti-apartheid, multi-racial political groups. The UDF publicly opposes violence.

Specifically, the South African government announced the following:

A ban on the receipt by UDF offunds or material aid from outside South Africa. Under a rarely invoked 1974 statute, the government yesterday declared the UDF an ``affected'' organization. The label is meant to apply to groups influenced by political groups from abroad. The South African authorities have long charged that the UDF is a mere front organization for the ANC, which has been outlawed since 1960.

A decision to oust the nearly 100,000 black laborers who have come from Mozambique, a country South Africa charges is supporting the ANC and contributing to land-mine attacks inside the South African border.

The migrant-labor ruling will not mean the immediate expulsion of the thousands of Mozambican blacks here, most of whom work in gold or coal mines. The government says that as the laborers' contracts expire during the next year, they will not be renewed. No new Mozambican workers will be allowed in.

Officials have stressed that the action is not intended as an economic ``counter-sanction,'' despite hints before last week's US sanctions vote that South Africa might deflect the effects of sanctions to neighboring black states.

The government indicated the expulsion was in reply to a series of land-mine explosions in northern regions of the country, near the Mozambican border. This is farm country, the heartland of the government's white political rivals on the extreme right.

Some 15 people have been killed in the mine blasts, and early this week six South African soldiers were wounded in a blast near the border. The government has charged the ANC with responsibility for the explosions.

The move against the UDF could deal a new blow to an organization already hurt by the arrest of thousands of its members under the state-of-emergency conditions. UDF sources say much of the organization's operating funds comes from outside South Africa, but deny government charges that it is a front for the ANC or any other organization.

Yesterday's ruling gives the government the right to start an investigation of the UDF and to petition the courts for permission to seize monies originating overseas. Some UDF activists are concerned that the move might foreshadow the formal banning of the organization, a move white conservatives have advocated.

The UDF said yesterday that it would fight the government move in court. But, under the emergency, the UDF has been hard-pressed to maintain the political momentum built up since its founding three years ago.

Many of the UDF activists are behind bars. Although UDF President Albertina Sisulu and founding ``patron'' Alan Boesak have not been detained, the state of emergency has kept the UDF from convening any large public meetings for quite some time.

Less stymied by the emergency have been labor unions. The National Union of Mineworkers, the country's largest black union, has hinted at strike action if the government follows through with the announced ouster of Mozambican workers.

This report was filed under South Africa's emergency regulations, which prohibit reporters from being ``within sight'' of any unrest, any ``restricted gathering,'' or any ``police actions''; from reporting on arrests made under the emergency regulations; and from relaying information deemed subversive.